Two doors with two guards - one lies, one tells the truth

You are a prisoner in a room with 2 doors and 2 guards. One of the doors will guide you to freedom and behind the other is a hangman --you don't know which is which.

One of the guards always tells the truth and the other always lies. You don't know which one is the truth-teller or the liar either.

You have to choose and open one of these doors, but you can only ask a single question to one of the guards.

What do you ask so you can pick the door to freedom?

edited Jun 29 '17 at 2:42

Wen1now

6,79822579

asked Sep 2 '14 at 19:13

Rafe

2,94432244

$begingroup$
Links to Wikipedia and TVTropes pages for this classic old puzzle.
$endgroup$
– Trevor Powell
Mar 12 '15 at 11:29

20

$begingroup$
And the obligatory XKCD link.
$endgroup$
– Trevor Powell
Mar 12 '15 at 13:13

1

$begingroup$
Why not just ask them something you know the answer to, like are you a guard? and then just listen to the one who says yes.
$endgroup$
– Ali Habibzadeh
Apr 18 '15 at 9:43

8

$begingroup$
@XGreen Because then you've used your one question and don't have any information about which door to take.
$endgroup$
– Kevin Stricker
Jun 9 '15 at 5:50

1

$begingroup$
This reminded me an old episode of Yu-Gi-Oh!, with Yûgi and Joey against the Paradox twin brothers !
$endgroup$
– Mayo
May 23 '16 at 14:37

|
show 2 more comments

You are a prisoner in a room with 2 doors and 2 guards. One of the doors will guide you to freedom and behind the other is a hangman --you don't know which is which.

One of the guards always tells the truth and the other always lies. You don't know which one is the truth-teller or the liar either.

You have to choose and open one of these doors, but you can only ask a single question to one of the guards.

What do you ask so you can pick the door to freedom?

edited Jun 29 '17 at 2:42

Wen1now

6,79822579

asked Sep 2 '14 at 19:13

Rafe

2,94432244

$begingroup$
Links to Wikipedia and TVTropes pages for this classic old puzzle.
$endgroup$
– Trevor Powell
Mar 12 '15 at 11:29

20

$begingroup$
And the obligatory XKCD link.
$endgroup$
– Trevor Powell
Mar 12 '15 at 13:13

1

$begingroup$
Why not just ask them something you know the answer to, like are you a guard? and then just listen to the one who says yes.
$endgroup$
– Ali Habibzadeh
Apr 18 '15 at 9:43

8

$begingroup$
@XGreen Because then you've used your one question and don't have any information about which door to take.
$endgroup$
– Kevin Stricker
Jun 9 '15 at 5:50

1

$begingroup$
This reminded me an old episode of Yu-Gi-Oh!, with Yûgi and Joey against the Paradox twin brothers !
$endgroup$
– Mayo
May 23 '16 at 14:37

|
show 2 more comments

You are a prisoner in a room with 2 doors and 2 guards. One of the doors will guide you to freedom and behind the other is a hangman --you don't know which is which.

One of the guards always tells the truth and the other always lies. You don't know which one is the truth-teller or the liar either.

You have to choose and open one of these doors, but you can only ask a single question to one of the guards.

What do you ask so you can pick the door to freedom?

edited Jun 29 '17 at 2:42

Wen1now

6,79822579

asked Sep 2 '14 at 19:13

Rafe

2,94432244

You are a prisoner in a room with 2 doors and 2 guards. One of the doors will guide you to freedom and behind the other is a hangman --you don't know which is which.

One of the guards always tells the truth and the other always lies. You don't know which one is the truth-teller or the liar either.

You have to choose and open one of these doors, but you can only ask a single question to one of the guards.

What do you ask so you can pick the door to freedom?

logical-deduction liars

edited Jun 29 '17 at 2:42

Wen1now

6,79822579

asked Sep 2 '14 at 19:13

Rafe

2,94432244

edited Jun 29 '17 at 2:42

Wen1now

6,79822579

asked Sep 2 '14 at 19:13

Rafe

2,94432244

edited Jun 29 '17 at 2:42

Wen1now

6,79822579

edited Jun 29 '17 at 2:42

Wen1now

6,79822579

edited Jun 29 '17 at 2:42

Wen1now

6,79822579

asked Sep 2 '14 at 19:13

Rafe

2,94432244

asked Sep 2 '14 at 19:13

Rafe

2,94432244

asked Sep 2 '14 at 19:13

Rafe

2,94432244

$begingroup$
Links to Wikipedia and TVTropes pages for this classic old puzzle.
$endgroup$
– Trevor Powell
Mar 12 '15 at 11:29

20

$begingroup$
And the obligatory XKCD link.
$endgroup$
– Trevor Powell
Mar 12 '15 at 13:13

1

$begingroup$
Why not just ask them something you know the answer to, like are you a guard? and then just listen to the one who says yes.
$endgroup$
– Ali Habibzadeh
Apr 18 '15 at 9:43

8

$begingroup$
@XGreen Because then you've used your one question and don't have any information about which door to take.
$endgroup$
– Kevin Stricker
Jun 9 '15 at 5:50

1

$begingroup$
This reminded me an old episode of Yu-Gi-Oh!, with Yûgi and Joey against the Paradox twin brothers !
$endgroup$
– Mayo
May 23 '16 at 14:37

|
show 2 more comments

$begingroup$
Links to Wikipedia and TVTropes pages for this classic old puzzle.
$endgroup$
– Trevor Powell
Mar 12 '15 at 11:29

20

$begingroup$
And the obligatory XKCD link.
$endgroup$
– Trevor Powell
Mar 12 '15 at 13:13

1

$begingroup$
Why not just ask them something you know the answer to, like are you a guard? and then just listen to the one who says yes.
$endgroup$
– Ali Habibzadeh
Apr 18 '15 at 9:43

8

$begingroup$
@XGreen Because then you've used your one question and don't have any information about which door to take.
$endgroup$
– Kevin Stricker
Jun 9 '15 at 5:50

1

$begingroup$
This reminded me an old episode of Yu-Gi-Oh!, with Yûgi and Joey against the Paradox twin brothers !
$endgroup$
– Mayo
May 23 '16 at 14:37

Links to Wikipedia and TVTropes pages for this classic old puzzle.

– Trevor Powell
Mar 12 '15 at 11:29

And the obligatory XKCD link.

– Trevor Powell
Mar 12 '15 at 13:13

Why not just ask them something you know the answer to, like are you a guard? and then just listen to the one who says yes.

– Ali Habibzadeh
Apr 18 '15 at 9:43

@XGreen Because then you've used your one question and don't have any information about which door to take.

– Kevin Stricker
Jun 9 '15 at 5:50

This reminded me an old episode of Yu-Gi-Oh!, with Yûgi and Joey against the Paradox twin brothers !

– Mayo
May 23 '16 at 14:37

|
show 2 more comments

16 Answers
16

active

oldest

votes

If I asked what door would lead to freedom, what door would the other guard point to?

If you asked the truth-guard, the truth-guard would tell you that the liar-guard would point to the door that leads to death.

If you asked the liar-guard, the liar-guard would tell you that the truth-guard would point to the door that leads to death.

Therefore, no matter who you ask, the guards tell you which door leads to death, and therefore you can pick the other door.

edited Jan 10 '17 at 5:59

Community♦

answered Sep 2 '14 at 22:25

Aza

10.3k84788

8

$begingroup$
This assumes that the liar guard will only tell a lie. (As opposed to trying to deceive you.) There is more to lying than words - there is body language.
$endgroup$
– Mayo
May 5 '15 at 18:56

16

$begingroup$
@Mayo It's a logic puzzle, though, not one of human interaction.
$endgroup$
– Aza
May 5 '15 at 19:53

1

$begingroup$
Yes. I forgot my :-) I've been thinking about questions like this along the line of a prisoner's dilemma and not only as a matter of a logic puzzle.
$endgroup$
– Mayo
May 5 '15 at 20:06

1

$begingroup$
@Mayo That makes sense! No worries :]
$endgroup$
– Aza
May 5 '15 at 20:13

3

$begingroup$
Note to self - if this answer is confusing, focus only on the latter clause which is simple to understand and the only key to the problem, i.e. What door would the other guard point to? (if you asked him where the treasure is). The reason this works is because the answer reveals the integrity of BOTH guards.
$endgroup$
– Sridhar-Sarnobat
Aug 13 '15 at 20:20

|
show 7 more comments

Choose a guard and ask him,

"If I asked you 'are you standing in front of the freedom door?', would your reply be 'yes'?"

If you choose the truthful guard, he will give you an honest answer. Enter his door if he says "yes" and enter the other door otherwise.

If you choose the liar, he will lie about what his reply would be. Since that reply is also a lie, the two lies cancel out. Enter his door if he says "yes" and enter the other door otherwise.

edited May 23 '16 at 14:06

manshu

5,2581553

answered Sep 2 '14 at 19:39

Kevin

5,71912330

8

$begingroup$
I think there was no information about where the guards stand. But that was a brilliant idea :-)
$endgroup$
– Rafe
Sep 3 '14 at 5:21

8

$begingroup$
That's true. But any question that distinguishes between the doors would work as the "inner" question. For example, 'is the door on the left the freedom door?'
$endgroup$
– Kevin
Sep 3 '14 at 11:54

6

$begingroup$
@TanSiongZhe, How so? Looks like a 100% chance to me. 1. If you ask the honest guard and he says "yes", you pass through his door safely. 2. If you ask the honest guard and he says "no", you pass through the other door safely. 3. If you ask the liar guard and he says "yes", you pass through his door safely. 4. If you ask the liar guard and he says "no", you pass through the other door safely. Where's the danger?
$endgroup$
– Kevin
Oct 23 '14 at 12:03

8

$begingroup$
The goal is not to find out which guard is truthful. The goal is to find the correct door. None of the other answers determine the identity of the guards either.
$endgroup$
– Kevin
Oct 24 '14 at 12:04

3

$begingroup$
I like @Emrakul's answer slightly more, because in real life, the lying guard will just want you to have fake information, and he won't care about "double negation" from his own answer. He will just tell you something so that you have fake information
$endgroup$
– 太極者無極而生
May 30 '15 at 19:27

|
show 5 more comments

Here is a twisted solution.

Go to any guard, point at a door and ask:

Among the propositions 1. "You are a liar", 2. "You will reply negatively" and 3. "This door leads to freedom", is there an odd number of true propositions?

If you get the answer yes:

If the guard is a truthteller, the number of truths is odd, 1. is false, 2. is false, so 3. must be true.

If the guard is a liar, the number of truths is even, 1. is true, 2. is false, so 3. must be true.

If you get a negative answer:

If the guard is a truthteller, the number of truths is even, 1. is false, 2. is true, so 3. must be true.

If the guard is a liar, the number of truths is odd, 1. is true, 2. is true, so 3. must be true.

So regardless of the answer of the guard, the door you pointed at is the door to freedom, you can leave safely.

Note:
Before you argue about this solution, please read the following:
Logic explanation in "two doors" answer

edited Apr 13 '17 at 12:50

Community♦

answered Sep 20 '14 at 9:39

Florian F

9,21612260

4

$begingroup$
What happens if you point to the door to death?
$endgroup$
– Rob Watts
Sep 20 '14 at 18:57

1

$begingroup$
Logic tells that doesn't happen. [smiley]
$endgroup$
– Florian F
Sep 20 '14 at 21:45

1

$begingroup$
Fantastic! This "twisted" solution of yours was truly a nice way to solve problems like this. Somehow removed the limit of questions
$endgroup$
– Rafe
Jan 15 '15 at 19:19

1

$begingroup$
This answer appears to be incorrect. The reasons have been stated in this question
$endgroup$
– March Ho
Jan 19 '15 at 13:31

1

$begingroup$
I know :-). If you point at the right door it works fine. If you point at the wrong door it results in a paradox as wnen one says "I am lying". It always brings a contradiction. That is why only the 1st possibility looks valid.
$endgroup$
– Florian F
Jan 19 '15 at 14:02

|
show 8 more comments

This is a classic old chestnut of a puzzle, and we've had several responses giving the traditional answer to the puzzle.
As a bit of spice, here's a slightly lateral-thinking answer, which lets us figure out not only which door is which, but also which guard is which, all using a single question.

We're going to be using a timing attack. Here's how it works.

Ask one of the guards:

"If I was to ask you what you'd say if I asked you what you'd say if I asked you what you'd say if I asked you what you'd say if I asked you what you'd say... (repeat any sufficiently large number of times, remembering to occasionally stop to draw a fresh lungful of air so that you don't faint) ...if I asked you whether this was the door to freedom, then what would you say your answer would be?"

The truth-telling guard will be able to answer this right away. No matter how many nested self-referential clauses you put into that question, she doesn't need to remember them or count them, and it always remains a trivial question to which she can always instantly give a truthful answer.

It's a much harder problem for the lying guard, who will need to think for a short time as she double-checks her answer, and may even have to ask you to repeat the question, just to make sure she correctly counted the number of recursive lies she's supposed to be telling, in order to give you the correct "always lies" answer. Her answer will necessarily be delayed, compared against the truth-teller's answer, because the question is designed to be much more difficult to determine the answer, for someone who is required to tell falsehoods.

Thus, simply by checking whether the answer is instantaneous or not, you can tell whether the guard is a truth-teller or a liar (respectively), and therefore select a door according to the answer that the guard gave you, and whether that guard is the truth-teller or the liar.

edited Sep 13 '18 at 15:52

Yout Ried

1,244624

answered May 31 '15 at 3:44

Trevor Powell

4,50821526

$begingroup$
That's a pretty good answer!
$endgroup$
– Xandawesome
May 31 '15 at 5:27

1

$begingroup$
It isn't that hard for the liar, he just has to count the the number of nested clauses. If even, he answers the correct door, else he points to the wrong one.
$endgroup$
– Rohcana
Aug 19 '15 at 18:05

$begingroup$
If the liar knows in advance that this is what you're going to do, sure. It'd be reasonably trivial to insert a confounding clause in the middle or end of the question such that merely counting the number of clauses won't work, if you're concerned about such a time-traveling (or puzzling.SE-reading) guard. :)
$endgroup$
– Trevor Powell
Aug 19 '15 at 23:28

7

$begingroup$
I'm afraid that if I was the truth telling guard, I'd still have to have a bit of a think before I'd answer that :)
$endgroup$
– Michał T
Jan 23 '16 at 23:57

$begingroup$
What if the liar guard is either a logical machine or omniscient and answers the question instantaneously? It takes a computer only a fraction of a second to evaluate the answer to !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!false (which is true, by the way).
$endgroup$
– Braden Best
May 24 '17 at 2:18

|
show 5 more comments

My answer, assuming I ask one question only...

Is the liar in front of the death door (DD)?

A. (if the Truth guard is asked) If the Truth guy is in front of DD, he answers NO. If not, he answers YES.

B. (if the Liar guard is asked) If the Liar is in front of DD, he answers NO. If not, he answers YES.

Either way, if we get a NO, then we've asked the guard in front of the death door, so we go to the opposite door. If we get a YES, then we've asked the guard not in front of the death door, so we go to the door behind them.

Thank you, I am here all week ;)

edited Dec 15 '18 at 8:57

ManyPinkHats

6,17922847

answered Dec 30 '14 at 3:45

user3035350

17913

add a comment |

EDIT: As @Ben mentioned, this answer is not really matching the logic-puzzle tag. Sorry.

If the guards are...

~~stupid~~ polite

you can

ask one to open a door

so you

didn't open a door (yet)

but

you see what's behind the door the guard opened

and

you can choose the right one (freedom, probably).

This doesn't even use the "truth-telling, lying" fact.

This only works if

The guards do what you say.

You can see freedom / the hangman when the door is open.

edited Jun 14 '16 at 15:39

answered Jun 14 '16 at 15:32

palsch

616715

3

$begingroup$
The OP set a tag called logic-puzzle. This is not a logical answer as you are locked up and they are guarding you from escaping. Please respect the question. (Sorry for being a bit harsh :-)). DISCLAIMER - THIS IS A SUBJECTIVE OPINION OF MINE
$endgroup$
– Ben
Jun 14 '16 at 15:36

$begingroup$
@Ben I'm sorry, I just thing lateral. lateral-thinking anywhere. Perhaps this is a problem of mine.
$endgroup$
– palsch
Jun 14 '16 at 15:38

$begingroup$
fair enough :-) no worries
$endgroup$
– Ben
Jun 14 '16 at 15:39

$begingroup$
Interesting idea, but how would you ask them? You: "Could you please open door 1?" Guard: Yes. (And stays standing there because while he can open the door, he won't.)
$endgroup$
– user1361991
Jun 14 '16 at 16:31

3

$begingroup$
@palsch Hmmm along that line of thinking: "Would you be a good chap and fetch the hangman for me?" should get the guard off in the right direction ;).
$endgroup$
– user1361991
Jun 14 '16 at 17:43

|
show 2 more comments

You ask:

Would the other guard say your door is the door to freedom?

If either guard is guarding the door to freedom both would answer NO.
If either guard is guarding the door to death both would answer YES.
NO leads to freedom and YES leads to death.

I don't think there is one question whose answer would indicate which guard
is guarding which door and which door leads to freedom.

edited Sep 14 '18 at 4:16

Yout Ried

1,244624

answered Jan 25 '15 at 17:51

Jim-EIL

5113

add a comment |

I am a newbie to Stack-Exchange and if my answer violates some guidelines , please correct me .

Let us assume , WLOG that the right door leads to freedom . So , the question I would ask is :

If I were to ask you whether the right door leads to freedom , would you answer yes?

The honest person would answer YES

The liar's internal response would be to say NO but being the liar he is , he would

say YES (due to double negation)

I think this would help you ascertain as to which is the truth-teller and which is the liar

edited May 23 '16 at 14:09

manshu

5,2581553

answered Dec 24 '14 at 5:04

pranav

5672720

2

$begingroup$
This answer is identical to (or at best a rehash of) Kevin's answer.
$endgroup$
– March Ho
Dec 30 '14 at 11:34

$begingroup$
@MarchHo Well, some people tend to not check whether their answer is already there, in order to avoid spoilers. Chances are both had the same idea independently that way.
$endgroup$
– Egor Hans
Sep 15 '18 at 20:32

$begingroup$
Would, "If I were to ask you if the wrong door leads to the hangman, would you answer no?", also do?
$endgroup$
– Martin Rattigan
Feb 27 at 23:57

add a comment |

I'd like to add an explanation for the answer of Florian F:

If we look at it from the other side, the possibilities are the following:

The door is correct and the guard tells the truth: Then 1 is false, 3 is true, 2 depends on how the guide answers the whole (true if no, false if yes), which fits in both cases (011= no, 001 = yes) => the guard can answer in any way and will pick one answer.

The door is correct and the guard lies: Then 1 is true, 3 is true, 2 is same as above, which in both cases is a lie (111=no=>yes, 101=yes=>no) => the guard can answer in any way and will pick one answer.

The door is not correct and the guard tells the truth: Then 1 is false, 3 is false, 2 is same again, but this time it does not fit (010=yes >< 2 true, 000=no >< 2 false) => the guard cannot answer.

The door is not correct and the guard lies: Then 1 is true, 3 is false, 2 is same again, but this time it's the truth (110=no=>yes -- 2 false, 100=yes=>no -- 2 true) => the guard cannot answer.

Thus, the reasoning is: If the guard can answer then you pointed at the freedom door, if it cannot answer you pointed at the death door.

However, this has one big danger, similar to the Halting Problem: You have no idea whether the guard really cannot answer, or if it's merely thinking about the decision yet what answer to pick, which may take quite a time! And additionally, every answer is assuming that if the guard answers, it answers as soon as it can. While for most answers this is not very relevant, for this answer a negation of this assumption would be fatal!

To illustrate this problem, the following example: Imagine you ask a guard the question Florian proposed. You wait, and wait, and wait. After a long time you decide that the guard doesn't answer because it can't. You then go to the door you didn't point at, and right when you just pushed down the handle, you hear the guard answering.

It was at this moment that Florian knew: He fucked up.

answered Jun 14 '16 at 15:13

Egor Hans

756

$begingroup$
If, however, there's a way to rather safely tell the guard cannot answer, then you're fine.
$endgroup$
– Egor Hans
Sep 15 '18 at 20:54

add a comment |

Emrakul provided the only answer that is close to the way I would explain this riddle solution. Others are violating one or more the original premise or just off base in general. Especially Kevin and Rafe which obviously just convinced themselves 50-50 was ok, LOL, which guard is the honest one and which is the liar is obviously not something you know, it says right in the riddle explanation, but your answer depends on it. I'm really scratching my head and wish you two the best.

The answer, same concept as Emrakul, in other words: Go to either guard. Ask that guard, "Which door would the other guard say is the safe route". This answer, regardless of which guard you ask will allow you to choose the door opposite of either guard's answer and always lead you safely. -1 x -1 = 1
MGO

answered Jan 23 '15 at 22:37

Matt O'Brien

391

add a comment |

As with all such questions you could try:

If the question Q (in this case, "Is this the dodgy door") were to be answered with the same truth or falsehood as you are about to answer this question, would the answer be "yes" (or maybe "pish" if you can remember that this is a native word for "yes" or "no" without exactly recalling which)?

If he answers "yes" (or "pish") then it is the dodgy door irrespective of whether he's a truth teller, a liar, a take your pick, or whether he actually knows the answer (and, where applicable, whether "pish" means yes or no). If he answers no (or "tush") then it isn't.

edited Feb 8 '17 at 5:02

answered Feb 8 '17 at 4:53

Martin Rattigan

1739

$begingroup$
This solution is actually surprisingly hard to analyze..
$endgroup$
– Egor Hans
Sep 15 '18 at 20:28

add a comment |

Update: My answer is wrong.

If anyone gets confused like I did, please see the comments for a reminder.

(original)

All of the answers are incorrect, and the puzzle itself flawed.

To reiterate the (overlooked) requirements:

"... you can only ask a single question from one of the
guards", and;

"One of the guards always tells the truth and the
other always lies. You don't know which one is [which]."

These rules create a situation where there is no method for discerning one guards' honesty from the other, and thus which door leads to freedom.

It's best explained as, "take my word for it*.

Edit - For Example, you ...

Ask Guard-A : "Do you guard the door to Freedom?" |OR| "Does Guard-B guard the door to freedom?"

Guard-A replies: "Yes"

50% - Guard-A is lying, you die.
50% - Guard-A is honest, you live.

edited Nov 14 '14 at 15:22

answered Nov 14 '14 at 10:10

Koffy

1173

3

$begingroup$
You don't have to discern the guards - you have to discern the doors. If you try all possible situations with the top two answers, you'll see you'll always pick the right door. Both ask one question from one guard who may or may not be the truthteller.
$endgroup$
– EagleV_Attnam
Nov 14 '14 at 10:26

$begingroup$
The answers are misleading because they immediately state "The honest-guard .. blah". How can you possible know which guard is which without asking both. See? Otherwise, just ask the honest guard and be done with the puzzle...
$endgroup$
– Koffy
Nov 14 '14 at 11:52

5

$begingroup$
@Koffy no, they start "If you asked the truth-guard," and go on to say "If you asked the liar-guard," thus demonstrating that you can safely act on the answer no matter which guard you happen to ask.
$endgroup$
– Kate Gregory
Nov 14 '14 at 13:09

2

$begingroup$
I can see my error now, thanks. I was never properly separating the 'defined behavior' from 'perceived behavior'. All in my head, the liar-guard would break behavior to .. achieve confusion. I even drew it on paper! I won't change my answer, but will add a note :) ty
$endgroup$
– Koffy
Nov 14 '14 at 15:16

add a comment |

It's common that you might find some difference in 2 doors.Let assume the colors of door are different,for say red & green.
So the prisoner chooses one door with a color red(for say) and asks this question to one of the guards.
"Is this blue door the road to my freedom".
If he asks this question to the truth speaking guard he will correct the prisoner over the color of the gate and will tell the true answer.
Instead if he asks to the false speaking guard he will not rectify his false statement over color of the door and so he will tell his wrong answer.

Difference in color is mere an assumption,some other difference in gates could also be used in the question.

answered Jul 22 '16 at 12:45

Ayush Srivastava

651

1

$begingroup$
what if the guard doesn't say anything about correctness of your question, and answers like this: "the door that you are pointing will lead you to freedom"?
$endgroup$
– Rafe
Jul 23 '16 at 6:18

add a comment |

Even though the most common answer has been posted I would like to add an answer that a friend of mine gave me when I asked him this question. I had never heard this answer before and this is why I would like to share it.

If you ask any guard, "is the truth telling guard standing in front of the door that leads to freedom?", if he says no you always go to the opposite door. If he says yes you simply go through that door.

The mechanics are more or less the same in this answer but for some reason it took me a long time to convince myself it worked every time.

edited Jun 29 '17 at 15:23

answered Jun 29 '17 at 15:13

Alex Bochel

267

$begingroup$
Ask the lieing guard that question, s/he will say no if the truth telling guard is in front of the door to freedom, taking the opposite door means you die. However which door the guard was referring to?
$endgroup$
– Matt Stevens
Feb 10 '18 at 11:22

1

$begingroup$
@MattStevens The answer actually does make sense. What you probably overlooked is that if you ask the liar, you refer to the door he's not standing in front of. That means that it fits: First, let's look at the scenario that you ask the guy in front of the freedom door. If you ask the truth-teller, the door behind him is the freedom door, and he says yes. If you ask the liar, the door behind him is the freedom door, so the door behind the other one (the truth-teller!) is death, which the liar negates to yes. Similar for the guard at the death door, who always says no.
$endgroup$
– Egor Hans
Jun 26 '18 at 13:01

add a comment |

You could also ask, "Why are there screams coming from behind this door?".

answered 18 mins ago

Martin Rattigan

1739

$begingroup$
This does not provide an answer to the question. To critique or request clarification from an author, leave a comment below their post. - From Review
$endgroup$
– F1Krazy
55 secs ago

add a comment |

-2

The answer to this riddle is to merely pose an single interogative that completes a simple alternating composite of positive and negative interdependent propositions. By stringing the propositions you can determine the door to freedom by posing the question so that the answer successfully or unsuccessfully completes the alternating polarity. Regardless of how you pose the question the polarity must alternate [negative, positive, negative with the answer providing the last swing back to positive] or [positive, negative, positive with the answer providing the last swing back to negative] to be successful. If and when it does not alternate, that will indicate the required action just as definitively as when and if it does successfully alternate.
Below, I will pose the question with the corresponding polarity; keep in mind that your question can utilize either door and either guard; to wit:
- Would you be telling the truth [positive], if you said that he would be lying [negative], if he said that this door is the door to freedom [positive]? Notice, [positive, negative, positive]. If the guard you directed the question to answers "NO" [negative], you have completed the correct alternating polarity, [positive, negative, positive,negative] and the door to freedom is the door you did not point out.

or you could ask it in the opposite fashion; to wit:
- Would you be lying [negative], if you said that he was telling the truth [positive], if he said that this is the door to certain death [negative]? Notice [Negative, positive, negative]. If he answers Yes [positive], you have again completed the alternating polarity correctly and your door to freedom is the one you pointed out. [Negative, Positive, Negative, Positive]. In either case, if the polarity does not correctly alternate, you should proceed as indicated from the context of your interrogative. Simple

answered Mar 11 '15 at 14:29

VicFore

131

4

$begingroup$
Welcome to puzzling.SE. "Everything should be made as simple as possible, but not simpler." - Albert Einstein
$endgroup$
– ABcDexter
Jun 14 '16 at 15:34

add a comment |

protected by Community♦ Mar 14 '15 at 21:51

Thank you for your interest in this question.
Because it has attracted low-quality or spam answers that had to be removed, posting an answer now requires 10 reputation on this site (the association bonus does not count).

Would you like to answer one of these unanswered questions instead?

16 Answers
16

active

oldest

votes

16 Answers
16

active

oldest

votes

If I asked what door would lead to freedom, what door would the other guard point to?

If you asked the truth-guard, the truth-guard would tell you that the liar-guard would point to the door that leads to death.

If you asked the liar-guard, the liar-guard would tell you that the truth-guard would point to the door that leads to death.

Therefore, no matter who you ask, the guards tell you which door leads to death, and therefore you can pick the other door.

edited Jan 10 '17 at 5:59

Community♦

answered Sep 2 '14 at 22:25

Aza

10.3k84788

8

$begingroup$
This assumes that the liar guard will only tell a lie. (As opposed to trying to deceive you.) There is more to lying than words - there is body language.
$endgroup$
– Mayo
May 5 '15 at 18:56

16

$begingroup$
@Mayo It's a logic puzzle, though, not one of human interaction.
$endgroup$
– Aza
May 5 '15 at 19:53

1

$begingroup$
Yes. I forgot my :-) I've been thinking about questions like this along the line of a prisoner's dilemma and not only as a matter of a logic puzzle.
$endgroup$
– Mayo
May 5 '15 at 20:06

1

$begingroup$
@Mayo That makes sense! No worries :]
$endgroup$
– Aza
May 5 '15 at 20:13

3

$begingroup$
Note to self - if this answer is confusing, focus only on the latter clause which is simple to understand and the only key to the problem, i.e. What door would the other guard point to? (if you asked him where the treasure is). The reason this works is because the answer reveals the integrity of BOTH guards.
$endgroup$
– Sridhar-Sarnobat
Aug 13 '15 at 20:20

|
show 7 more comments

If I asked what door would lead to freedom, what door would the other guard point to?

If you asked the truth-guard, the truth-guard would tell you that the liar-guard would point to the door that leads to death.

If you asked the liar-guard, the liar-guard would tell you that the truth-guard would point to the door that leads to death.

Therefore, no matter who you ask, the guards tell you which door leads to death, and therefore you can pick the other door.

edited Jan 10 '17 at 5:59

Community♦

answered Sep 2 '14 at 22:25

Aza

10.3k84788

8

$begingroup$
This assumes that the liar guard will only tell a lie. (As opposed to trying to deceive you.) There is more to lying than words - there is body language.
$endgroup$
– Mayo
May 5 '15 at 18:56

16

$begingroup$
@Mayo It's a logic puzzle, though, not one of human interaction.
$endgroup$
– Aza
May 5 '15 at 19:53

1

$begingroup$
Yes. I forgot my :-) I've been thinking about questions like this along the line of a prisoner's dilemma and not only as a matter of a logic puzzle.
$endgroup$
– Mayo
May 5 '15 at 20:06

1

$begingroup$
@Mayo That makes sense! No worries :]
$endgroup$
– Aza
May 5 '15 at 20:13

3

$begingroup$
Note to self - if this answer is confusing, focus only on the latter clause which is simple to understand and the only key to the problem, i.e. What door would the other guard point to? (if you asked him where the treasure is). The reason this works is because the answer reveals the integrity of BOTH guards.
$endgroup$
– Sridhar-Sarnobat
Aug 13 '15 at 20:20

|
show 7 more comments

If I asked what door would lead to freedom, what door would the other guard point to?

If you asked the truth-guard, the truth-guard would tell you that the liar-guard would point to the door that leads to death.

If you asked the liar-guard, the liar-guard would tell you that the truth-guard would point to the door that leads to death.

Therefore, no matter who you ask, the guards tell you which door leads to death, and therefore you can pick the other door.

edited Jan 10 '17 at 5:59

Community♦

answered Sep 2 '14 at 22:25

Aza

10.3k84788

If I asked what door would lead to freedom, what door would the other guard point to?

If you asked the truth-guard, the truth-guard would tell you that the liar-guard would point to the door that leads to death.

If you asked the liar-guard, the liar-guard would tell you that the truth-guard would point to the door that leads to death.

Therefore, no matter who you ask, the guards tell you which door leads to death, and therefore you can pick the other door.

edited Jan 10 '17 at 5:59

Community♦

answered Sep 2 '14 at 22:25

Aza

10.3k84788

edited Jan 10 '17 at 5:59

Community♦

edited Jan 10 '17 at 5:59

Community♦

edited Jan 10 '17 at 5:59

Community♦

answered Sep 2 '14 at 22:25

Aza

10.3k84788

answered Sep 2 '14 at 22:25

Aza

10.3k84788

answered Sep 2 '14 at 22:25

Aza

10.3k84788

8

$begingroup$
This assumes that the liar guard will only tell a lie. (As opposed to trying to deceive you.) There is more to lying than words - there is body language.
$endgroup$
– Mayo
May 5 '15 at 18:56

16

$begingroup$
@Mayo It's a logic puzzle, though, not one of human interaction.
$endgroup$
– Aza
May 5 '15 at 19:53

1

$begingroup$
Yes. I forgot my :-) I've been thinking about questions like this along the line of a prisoner's dilemma and not only as a matter of a logic puzzle.
$endgroup$
– Mayo
May 5 '15 at 20:06

1

$begingroup$
@Mayo That makes sense! No worries :]
$endgroup$
– Aza
May 5 '15 at 20:13

3

$begingroup$
Note to self - if this answer is confusing, focus only on the latter clause which is simple to understand and the only key to the problem, i.e. What door would the other guard point to? (if you asked him where the treasure is). The reason this works is because the answer reveals the integrity of BOTH guards.
$endgroup$
– Sridhar-Sarnobat
Aug 13 '15 at 20:20

|
show 7 more comments

8

$begingroup$
This assumes that the liar guard will only tell a lie. (As opposed to trying to deceive you.) There is more to lying than words - there is body language.
$endgroup$
– Mayo
May 5 '15 at 18:56

16

$begingroup$
@Mayo It's a logic puzzle, though, not one of human interaction.
$endgroup$
– Aza
May 5 '15 at 19:53

1

$begingroup$
Yes. I forgot my :-) I've been thinking about questions like this along the line of a prisoner's dilemma and not only as a matter of a logic puzzle.
$endgroup$
– Mayo
May 5 '15 at 20:06

1

$begingroup$
@Mayo That makes sense! No worries :]
$endgroup$
– Aza
May 5 '15 at 20:13

3

$begingroup$
Note to self - if this answer is confusing, focus only on the latter clause which is simple to understand and the only key to the problem, i.e. What door would the other guard point to? (if you asked him where the treasure is). The reason this works is because the answer reveals the integrity of BOTH guards.
$endgroup$
– Sridhar-Sarnobat
Aug 13 '15 at 20:20

This assumes that the liar guard will only tell a lie. (As opposed to trying to deceive you.) There is more to lying than words - there is body language.

– Mayo
May 5 '15 at 18:56

@Mayo It's a logic puzzle, though, not one of human interaction.

– Aza
May 5 '15 at 19:53

Yes. I forgot my :-) I've been thinking about questions like this along the line of a prisoner's dilemma and not only as a matter of a logic puzzle.

– Mayo
May 5 '15 at 20:06

@Mayo That makes sense! No worries :]

– Aza
May 5 '15 at 20:13

Note to self - if this answer is confusing, focus only on the latter clause which is simple to understand and the only key to the problem, i.e. What door would the other guard point to? (if you asked him where the treasure is). The reason this works is because the answer reveals the integrity of BOTH guards.

– Sridhar-Sarnobat
Aug 13 '15 at 20:20

|
show 7 more comments

Choose a guard and ask him,

"If I asked you 'are you standing in front of the freedom door?', would your reply be 'yes'?"

If you choose the truthful guard, he will give you an honest answer. Enter his door if he says "yes" and enter the other door otherwise.

If you choose the liar, he will lie about what his reply would be. Since that reply is also a lie, the two lies cancel out. Enter his door if he says "yes" and enter the other door otherwise.

edited May 23 '16 at 14:06

manshu

5,2581553

answered Sep 2 '14 at 19:39

Kevin

5,71912330

8

$begingroup$
I think there was no information about where the guards stand. But that was a brilliant idea :-)
$endgroup$
– Rafe
Sep 3 '14 at 5:21

8

$begingroup$
That's true. But any question that distinguishes between the doors would work as the "inner" question. For example, 'is the door on the left the freedom door?'
$endgroup$
– Kevin
Sep 3 '14 at 11:54

6

$begingroup$
@TanSiongZhe, How so? Looks like a 100% chance to me. 1. If you ask the honest guard and he says "yes", you pass through his door safely. 2. If you ask the honest guard and he says "no", you pass through the other door safely. 3. If you ask the liar guard and he says "yes", you pass through his door safely. 4. If you ask the liar guard and he says "no", you pass through the other door safely. Where's the danger?
$endgroup$
– Kevin
Oct 23 '14 at 12:03

8

$begingroup$
The goal is not to find out which guard is truthful. The goal is to find the correct door. None of the other answers determine the identity of the guards either.
$endgroup$
– Kevin
Oct 24 '14 at 12:04

3

$begingroup$
I like @Emrakul's answer slightly more, because in real life, the lying guard will just want you to have fake information, and he won't care about "double negation" from his own answer. He will just tell you something so that you have fake information
$endgroup$
– 太極者無極而生
May 30 '15 at 19:27

|
show 5 more comments

Choose a guard and ask him,

"If I asked you 'are you standing in front of the freedom door?', would your reply be 'yes'?"

If you choose the truthful guard, he will give you an honest answer. Enter his door if he says "yes" and enter the other door otherwise.

If you choose the liar, he will lie about what his reply would be. Since that reply is also a lie, the two lies cancel out. Enter his door if he says "yes" and enter the other door otherwise.

edited May 23 '16 at 14:06

manshu

5,2581553

answered Sep 2 '14 at 19:39

Kevin

5,71912330

8

$begingroup$
I think there was no information about where the guards stand. But that was a brilliant idea :-)
$endgroup$
– Rafe
Sep 3 '14 at 5:21

8

$begingroup$
That's true. But any question that distinguishes between the doors would work as the "inner" question. For example, 'is the door on the left the freedom door?'
$endgroup$
– Kevin
Sep 3 '14 at 11:54

6

$begingroup$
@TanSiongZhe, How so? Looks like a 100% chance to me. 1. If you ask the honest guard and he says "yes", you pass through his door safely. 2. If you ask the honest guard and he says "no", you pass through the other door safely. 3. If you ask the liar guard and he says "yes", you pass through his door safely. 4. If you ask the liar guard and he says "no", you pass through the other door safely. Where's the danger?
$endgroup$
– Kevin
Oct 23 '14 at 12:03

8

$begingroup$
The goal is not to find out which guard is truthful. The goal is to find the correct door. None of the other answers determine the identity of the guards either.
$endgroup$
– Kevin
Oct 24 '14 at 12:04

3

$begingroup$
I like @Emrakul's answer slightly more, because in real life, the lying guard will just want you to have fake information, and he won't care about "double negation" from his own answer. He will just tell you something so that you have fake information
$endgroup$
– 太極者無極而生
May 30 '15 at 19:27

|
show 5 more comments

Choose a guard and ask him,

"If I asked you 'are you standing in front of the freedom door?', would your reply be 'yes'?"

If you choose the truthful guard, he will give you an honest answer. Enter his door if he says "yes" and enter the other door otherwise.

If you choose the liar, he will lie about what his reply would be. Since that reply is also a lie, the two lies cancel out. Enter his door if he says "yes" and enter the other door otherwise.

edited May 23 '16 at 14:06

manshu

5,2581553

answered Sep 2 '14 at 19:39

Kevin

5,71912330

Choose a guard and ask him,

"If I asked you 'are you standing in front of the freedom door?', would your reply be 'yes'?"

If you choose the truthful guard, he will give you an honest answer. Enter his door if he says "yes" and enter the other door otherwise.

If you choose the liar, he will lie about what his reply would be. Since that reply is also a lie, the two lies cancel out. Enter his door if he says "yes" and enter the other door otherwise.

edited May 23 '16 at 14:06

manshu

5,2581553

answered Sep 2 '14 at 19:39

Kevin

5,71912330

edited May 23 '16 at 14:06

manshu

5,2581553

edited May 23 '16 at 14:06

manshu

5,2581553

edited May 23 '16 at 14:06

manshu

5,2581553

answered Sep 2 '14 at 19:39

Kevin

5,71912330

answered Sep 2 '14 at 19:39

Kevin

5,71912330

answered Sep 2 '14 at 19:39

Kevin

5,71912330

8

$begingroup$
I think there was no information about where the guards stand. But that was a brilliant idea :-)
$endgroup$
– Rafe
Sep 3 '14 at 5:21

8

$begingroup$
That's true. But any question that distinguishes between the doors would work as the "inner" question. For example, 'is the door on the left the freedom door?'
$endgroup$
– Kevin
Sep 3 '14 at 11:54

6

$begingroup$
@TanSiongZhe, How so? Looks like a 100% chance to me. 1. If you ask the honest guard and he says "yes", you pass through his door safely. 2. If you ask the honest guard and he says "no", you pass through the other door safely. 3. If you ask the liar guard and he says "yes", you pass through his door safely. 4. If you ask the liar guard and he says "no", you pass through the other door safely. Where's the danger?
$endgroup$
– Kevin
Oct 23 '14 at 12:03

8

$begingroup$
The goal is not to find out which guard is truthful. The goal is to find the correct door. None of the other answers determine the identity of the guards either.
$endgroup$
– Kevin
Oct 24 '14 at 12:04

3

$begingroup$
I like @Emrakul's answer slightly more, because in real life, the lying guard will just want you to have fake information, and he won't care about "double negation" from his own answer. He will just tell you something so that you have fake information
$endgroup$
– 太極者無極而生
May 30 '15 at 19:27

|
show 5 more comments

8

$begingroup$
I think there was no information about where the guards stand. But that was a brilliant idea :-)
$endgroup$
– Rafe
Sep 3 '14 at 5:21

8

$begingroup$
That's true. But any question that distinguishes between the doors would work as the "inner" question. For example, 'is the door on the left the freedom door?'
$endgroup$
– Kevin
Sep 3 '14 at 11:54

6

$begingroup$
@TanSiongZhe, How so? Looks like a 100% chance to me. 1. If you ask the honest guard and he says "yes", you pass through his door safely. 2. If you ask the honest guard and he says "no", you pass through the other door safely. 3. If you ask the liar guard and he says "yes", you pass through his door safely. 4. If you ask the liar guard and he says "no", you pass through the other door safely. Where's the danger?
$endgroup$
– Kevin
Oct 23 '14 at 12:03

8

$begingroup$
The goal is not to find out which guard is truthful. The goal is to find the correct door. None of the other answers determine the identity of the guards either.
$endgroup$
– Kevin
Oct 24 '14 at 12:04

3

$begingroup$
I like @Emrakul's answer slightly more, because in real life, the lying guard will just want you to have fake information, and he won't care about "double negation" from his own answer. He will just tell you something so that you have fake information
$endgroup$
– 太極者無極而生
May 30 '15 at 19:27

I think there was no information about where the guards stand. But that was a brilliant idea :-)

– Rafe
Sep 3 '14 at 5:21

That's true. But any question that distinguishes between the doors would work as the "inner" question. For example, 'is the door on the left the freedom door?'

– Kevin
Sep 3 '14 at 11:54

@TanSiongZhe, How so? Looks like a 100% chance to me. 1. If you ask the honest guard and he says "yes", you pass through his door safely. 2. If you ask the honest guard and he says "no", you pass through the other door safely. 3. If you ask the liar guard and he says "yes", you pass through his door safely. 4. If you ask the liar guard and he says "no", you pass through the other door safely. Where's the danger?

– Kevin
Oct 23 '14 at 12:03

The goal is not to find out which guard is truthful. The goal is to find the correct door. None of the other answers determine the identity of the guards either.

– Kevin
Oct 24 '14 at 12:04

I like @Emrakul's answer slightly more, because in real life, the lying guard will just want you to have fake information, and he won't care about "double negation" from his own answer. He will just tell you something so that you have fake information

– 太極者無極而生
May 30 '15 at 19:27

|
show 5 more comments

Here is a twisted solution.

Go to any guard, point at a door and ask:

Among the propositions 1. "You are a liar", 2. "You will reply negatively" and 3. "This door leads to freedom", is there an odd number of true propositions?

So regardless of the answer of the guard, the door you pointed at is the door to freedom, you can leave safely.

Note:
Before you argue about this solution, please read the following:
Logic explanation in "two doors" answer

edited Apr 13 '17 at 12:50

Community♦

answered Sep 20 '14 at 9:39

Florian F

9,21612260

4

$begingroup$
What happens if you point to the door to death?
$endgroup$
– Rob Watts
Sep 20 '14 at 18:57

1

$begingroup$
Logic tells that doesn't happen. [smiley]
$endgroup$
– Florian F
Sep 20 '14 at 21:45

1

$begingroup$
Fantastic! This "twisted" solution of yours was truly a nice way to solve problems like this. Somehow removed the limit of questions
$endgroup$
– Rafe
Jan 15 '15 at 19:19

1

$begingroup$
This answer appears to be incorrect. The reasons have been stated in this question
$endgroup$
– March Ho
Jan 19 '15 at 13:31

1

$begingroup$
I know :-). If you point at the right door it works fine. If you point at the wrong door it results in a paradox as wnen one says "I am lying". It always brings a contradiction. That is why only the 1st possibility looks valid.
$endgroup$
– Florian F
Jan 19 '15 at 14:02

|
show 8 more comments

Here is a twisted solution.

Go to any guard, point at a door and ask:

Among the propositions 1. "You are a liar", 2. "You will reply negatively" and 3. "This door leads to freedom", is there an odd number of true propositions?

So regardless of the answer of the guard, the door you pointed at is the door to freedom, you can leave safely.

Note:
Before you argue about this solution, please read the following:
Logic explanation in "two doors" answer

edited Apr 13 '17 at 12:50

Community♦

answered Sep 20 '14 at 9:39

Florian F

9,21612260

4

$begingroup$
What happens if you point to the door to death?
$endgroup$
– Rob Watts
Sep 20 '14 at 18:57

1

$begingroup$
Logic tells that doesn't happen. [smiley]
$endgroup$
– Florian F
Sep 20 '14 at 21:45

1

$begingroup$
Fantastic! This "twisted" solution of yours was truly a nice way to solve problems like this. Somehow removed the limit of questions
$endgroup$
– Rafe
Jan 15 '15 at 19:19

1

$begingroup$
This answer appears to be incorrect. The reasons have been stated in this question
$endgroup$
– March Ho
Jan 19 '15 at 13:31

1

$begingroup$
I know :-). If you point at the right door it works fine. If you point at the wrong door it results in a paradox as wnen one says "I am lying". It always brings a contradiction. That is why only the 1st possibility looks valid.
$endgroup$
– Florian F
Jan 19 '15 at 14:02

|
show 8 more comments

Here is a twisted solution.

Go to any guard, point at a door and ask:

Among the propositions 1. "You are a liar", 2. "You will reply negatively" and 3. "This door leads to freedom", is there an odd number of true propositions?

So regardless of the answer of the guard, the door you pointed at is the door to freedom, you can leave safely.

Note:
Before you argue about this solution, please read the following:
Logic explanation in "two doors" answer

edited Apr 13 '17 at 12:50

Community♦

answered Sep 20 '14 at 9:39

Florian F

9,21612260

Here is a twisted solution.

Go to any guard, point at a door and ask:

Among the propositions 1. "You are a liar", 2. "You will reply negatively" and 3. "This door leads to freedom", is there an odd number of true propositions?

So regardless of the answer of the guard, the door you pointed at is the door to freedom, you can leave safely.

Note:
Before you argue about this solution, please read the following:
Logic explanation in "two doors" answer

edited Apr 13 '17 at 12:50

Community♦

answered Sep 20 '14 at 9:39

Florian F

9,21612260

edited Apr 13 '17 at 12:50

Community♦

edited Apr 13 '17 at 12:50

Community♦

edited Apr 13 '17 at 12:50

Community♦

answered Sep 20 '14 at 9:39

Florian F

9,21612260

answered Sep 20 '14 at 9:39

Florian F

9,21612260

answered Sep 20 '14 at 9:39

Florian F

9,21612260

4

$begingroup$
What happens if you point to the door to death?
$endgroup$
– Rob Watts
Sep 20 '14 at 18:57

1

$begingroup$
Logic tells that doesn't happen. [smiley]
$endgroup$
– Florian F
Sep 20 '14 at 21:45

1

$begingroup$
Fantastic! This "twisted" solution of yours was truly a nice way to solve problems like this. Somehow removed the limit of questions
$endgroup$
– Rafe
Jan 15 '15 at 19:19

1

$begingroup$
This answer appears to be incorrect. The reasons have been stated in this question
$endgroup$
– March Ho
Jan 19 '15 at 13:31

1

$begingroup$
I know :-). If you point at the right door it works fine. If you point at the wrong door it results in a paradox as wnen one says "I am lying". It always brings a contradiction. That is why only the 1st possibility looks valid.
$endgroup$
– Florian F
Jan 19 '15 at 14:02

|
show 8 more comments

4

$begingroup$
What happens if you point to the door to death?
$endgroup$
– Rob Watts
Sep 20 '14 at 18:57

1

$begingroup$
Logic tells that doesn't happen. [smiley]
$endgroup$
– Florian F
Sep 20 '14 at 21:45

1

$begingroup$
Fantastic! This "twisted" solution of yours was truly a nice way to solve problems like this. Somehow removed the limit of questions
$endgroup$
– Rafe
Jan 15 '15 at 19:19

1

$begingroup$
This answer appears to be incorrect. The reasons have been stated in this question
$endgroup$
– March Ho
Jan 19 '15 at 13:31

1

$begingroup$
I know :-). If you point at the right door it works fine. If you point at the wrong door it results in a paradox as wnen one says "I am lying". It always brings a contradiction. That is why only the 1st possibility looks valid.
$endgroup$
– Florian F
Jan 19 '15 at 14:02

What happens if you point to the door to death?

– Rob Watts
Sep 20 '14 at 18:57

Logic tells that doesn't happen. [smiley]

– Florian F
Sep 20 '14 at 21:45

Fantastic! This "twisted" solution of yours was truly a nice way to solve problems like this. Somehow removed the limit of questions

– Rafe
Jan 15 '15 at 19:19

This answer appears to be incorrect. The reasons have been stated in this question

– March Ho
Jan 19 '15 at 13:31

I know :-). If you point at the right door it works fine. If you point at the wrong door it results in a paradox as wnen one says "I am lying". It always brings a contradiction. That is why only the 1st possibility looks valid.

– Florian F
Jan 19 '15 at 14:02

|
show 8 more comments

This is a classic old chestnut of a puzzle, and we've had several responses giving the traditional answer to the puzzle.
As a bit of spice, here's a slightly lateral-thinking answer, which lets us figure out not only which door is which, but also which guard is which, all using a single question.

We're going to be using a timing attack. Here's how it works.

Ask one of the guards:

"If I was to ask you what you'd say if I asked you what you'd say if I asked you what you'd say if I asked you what you'd say if I asked you what you'd say... (repeat any sufficiently large number of times, remembering to occasionally stop to draw a fresh lungful of air so that you don't faint) ...if I asked you whether this was the door to freedom, then what would you say your answer would be?"

It's a much harder problem for the lying guard, who will need to think for a short time as she double-checks her answer, and may even have to ask you to repeat the question, just to make sure she correctly counted the number of recursive lies she's supposed to be telling, in order to give you the correct "always lies" answer. Her answer will necessarily be delayed, compared against the truth-teller's answer, because the question is designed to be much more difficult to determine the answer, for someone who is required to tell falsehoods.

edited Sep 13 '18 at 15:52

Yout Ried

1,244624

answered May 31 '15 at 3:44

Trevor Powell

4,50821526

$begingroup$
That's a pretty good answer!
$endgroup$
– Xandawesome
May 31 '15 at 5:27

1

$begingroup$
It isn't that hard for the liar, he just has to count the the number of nested clauses. If even, he answers the correct door, else he points to the wrong one.
$endgroup$
– Rohcana
Aug 19 '15 at 18:05

$begingroup$
If the liar knows in advance that this is what you're going to do, sure. It'd be reasonably trivial to insert a confounding clause in the middle or end of the question such that merely counting the number of clauses won't work, if you're concerned about such a time-traveling (or puzzling.SE-reading) guard. :)
$endgroup$
– Trevor Powell
Aug 19 '15 at 23:28

7

$begingroup$
I'm afraid that if I was the truth telling guard, I'd still have to have a bit of a think before I'd answer that :)
$endgroup$
– Michał T
Jan 23 '16 at 23:57

$begingroup$
What if the liar guard is either a logical machine or omniscient and answers the question instantaneously? It takes a computer only a fraction of a second to evaluate the answer to !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!false (which is true, by the way).
$endgroup$
– Braden Best
May 24 '17 at 2:18

|
show 5 more comments

This is a classic old chestnut of a puzzle, and we've had several responses giving the traditional answer to the puzzle.
As a bit of spice, here's a slightly lateral-thinking answer, which lets us figure out not only which door is which, but also which guard is which, all using a single question.

We're going to be using a timing attack. Here's how it works.

Ask one of the guards:

"If I was to ask you what you'd say if I asked you what you'd say if I asked you what you'd say if I asked you what you'd say if I asked you what you'd say... (repeat any sufficiently large number of times, remembering to occasionally stop to draw a fresh lungful of air so that you don't faint) ...if I asked you whether this was the door to freedom, then what would you say your answer would be?"

It's a much harder problem for the lying guard, who will need to think for a short time as she double-checks her answer, and may even have to ask you to repeat the question, just to make sure she correctly counted the number of recursive lies she's supposed to be telling, in order to give you the correct "always lies" answer. Her answer will necessarily be delayed, compared against the truth-teller's answer, because the question is designed to be much more difficult to determine the answer, for someone who is required to tell falsehoods.

edited Sep 13 '18 at 15:52

Yout Ried

1,244624

answered May 31 '15 at 3:44

Trevor Powell

4,50821526

$begingroup$
That's a pretty good answer!
$endgroup$
– Xandawesome
May 31 '15 at 5:27

1

$begingroup$
It isn't that hard for the liar, he just has to count the the number of nested clauses. If even, he answers the correct door, else he points to the wrong one.
$endgroup$
– Rohcana
Aug 19 '15 at 18:05

$begingroup$
If the liar knows in advance that this is what you're going to do, sure. It'd be reasonably trivial to insert a confounding clause in the middle or end of the question such that merely counting the number of clauses won't work, if you're concerned about such a time-traveling (or puzzling.SE-reading) guard. :)
$endgroup$
– Trevor Powell
Aug 19 '15 at 23:28

7

$begingroup$
I'm afraid that if I was the truth telling guard, I'd still have to have a bit of a think before I'd answer that :)
$endgroup$
– Michał T
Jan 23 '16 at 23:57

$begingroup$
What if the liar guard is either a logical machine or omniscient and answers the question instantaneously? It takes a computer only a fraction of a second to evaluate the answer to !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!false (which is true, by the way).
$endgroup$
– Braden Best
May 24 '17 at 2:18

|
show 5 more comments

This is a classic old chestnut of a puzzle, and we've had several responses giving the traditional answer to the puzzle.
As a bit of spice, here's a slightly lateral-thinking answer, which lets us figure out not only which door is which, but also which guard is which, all using a single question.

We're going to be using a timing attack. Here's how it works.

Ask one of the guards:

"If I was to ask you what you'd say if I asked you what you'd say if I asked you what you'd say if I asked you what you'd say if I asked you what you'd say... (repeat any sufficiently large number of times, remembering to occasionally stop to draw a fresh lungful of air so that you don't faint) ...if I asked you whether this was the door to freedom, then what would you say your answer would be?"

It's a much harder problem for the lying guard, who will need to think for a short time as she double-checks her answer, and may even have to ask you to repeat the question, just to make sure she correctly counted the number of recursive lies she's supposed to be telling, in order to give you the correct "always lies" answer. Her answer will necessarily be delayed, compared against the truth-teller's answer, because the question is designed to be much more difficult to determine the answer, for someone who is required to tell falsehoods.

edited Sep 13 '18 at 15:52

Yout Ried

1,244624

answered May 31 '15 at 3:44

Trevor Powell

4,50821526

This is a classic old chestnut of a puzzle, and we've had several responses giving the traditional answer to the puzzle.
As a bit of spice, here's a slightly lateral-thinking answer, which lets us figure out not only which door is which, but also which guard is which, all using a single question.

We're going to be using a timing attack. Here's how it works.

Ask one of the guards:

"If I was to ask you what you'd say if I asked you what you'd say if I asked you what you'd say if I asked you what you'd say if I asked you what you'd say... (repeat any sufficiently large number of times, remembering to occasionally stop to draw a fresh lungful of air so that you don't faint) ...if I asked you whether this was the door to freedom, then what would you say your answer would be?"

It's a much harder problem for the lying guard, who will need to think for a short time as she double-checks her answer, and may even have to ask you to repeat the question, just to make sure she correctly counted the number of recursive lies she's supposed to be telling, in order to give you the correct "always lies" answer. Her answer will necessarily be delayed, compared against the truth-teller's answer, because the question is designed to be much more difficult to determine the answer, for someone who is required to tell falsehoods.

edited Sep 13 '18 at 15:52

Yout Ried

1,244624

answered May 31 '15 at 3:44

Trevor Powell

4,50821526

edited Sep 13 '18 at 15:52

Yout Ried

1,244624

edited Sep 13 '18 at 15:52

Yout Ried

1,244624

edited Sep 13 '18 at 15:52

Yout Ried

1,244624

answered May 31 '15 at 3:44

Trevor Powell

4,50821526

answered May 31 '15 at 3:44

Trevor Powell

4,50821526

answered May 31 '15 at 3:44

Trevor Powell

4,50821526

$begingroup$
That's a pretty good answer!
$endgroup$
– Xandawesome
May 31 '15 at 5:27

1

$begingroup$
It isn't that hard for the liar, he just has to count the the number of nested clauses. If even, he answers the correct door, else he points to the wrong one.
$endgroup$
– Rohcana
Aug 19 '15 at 18:05

$begingroup$
If the liar knows in advance that this is what you're going to do, sure. It'd be reasonably trivial to insert a confounding clause in the middle or end of the question such that merely counting the number of clauses won't work, if you're concerned about such a time-traveling (or puzzling.SE-reading) guard. :)
$endgroup$
– Trevor Powell
Aug 19 '15 at 23:28

7

$begingroup$
I'm afraid that if I was the truth telling guard, I'd still have to have a bit of a think before I'd answer that :)
$endgroup$
– Michał T
Jan 23 '16 at 23:57

$begingroup$
What if the liar guard is either a logical machine or omniscient and answers the question instantaneously? It takes a computer only a fraction of a second to evaluate the answer to !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!false (which is true, by the way).
$endgroup$
– Braden Best
May 24 '17 at 2:18

|
show 5 more comments

$begingroup$
That's a pretty good answer!
$endgroup$
– Xandawesome
May 31 '15 at 5:27

1

$begingroup$
It isn't that hard for the liar, he just has to count the the number of nested clauses. If even, he answers the correct door, else he points to the wrong one.
$endgroup$
– Rohcana
Aug 19 '15 at 18:05

$begingroup$
If the liar knows in advance that this is what you're going to do, sure. It'd be reasonably trivial to insert a confounding clause in the middle or end of the question such that merely counting the number of clauses won't work, if you're concerned about such a time-traveling (or puzzling.SE-reading) guard. :)
$endgroup$
– Trevor Powell
Aug 19 '15 at 23:28

7

$begingroup$
I'm afraid that if I was the truth telling guard, I'd still have to have a bit of a think before I'd answer that :)
$endgroup$
– Michał T
Jan 23 '16 at 23:57

$begingroup$
What if the liar guard is either a logical machine or omniscient and answers the question instantaneously? It takes a computer only a fraction of a second to evaluate the answer to !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!false (which is true, by the way).
$endgroup$
– Braden Best
May 24 '17 at 2:18

That's a pretty good answer!

– Xandawesome
May 31 '15 at 5:27

It isn't that hard for the liar, he just has to count the the number of nested clauses. If even, he answers the correct door, else he points to the wrong one.

– Rohcana
Aug 19 '15 at 18:05

If the liar knows in advance that this is what you're going to do, sure. It'd be reasonably trivial to insert a confounding clause in the middle or end of the question such that merely counting the number of clauses won't work, if you're concerned about such a time-traveling (or puzzling.SE-reading) guard. :)

– Trevor Powell
Aug 19 '15 at 23:28

I'm afraid that if I was the truth telling guard, I'd still have to have a bit of a think before I'd answer that :)

– Michał T
Jan 23 '16 at 23:57

What if the liar guard is either a logical machine or omniscient and answers the question instantaneously? It takes a computer only a fraction of a second to evaluate the answer to

!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!false

(which is true, by the way).

– Braden Best
May 24 '17 at 2:18

What if the liar guard is either a logical machine or omniscient and answers the question instantaneously? It takes a computer only a fraction of a second to evaluate the answer to

!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!false

(which is true, by the way).

– Braden Best
May 24 '17 at 2:18

|
show 5 more comments

My answer, assuming I ask one question only...

Is the liar in front of the death door (DD)?

A. (if the Truth guard is asked) If the Truth guy is in front of DD, he answers NO. If not, he answers YES.

B. (if the Liar guard is asked) If the Liar is in front of DD, he answers NO. If not, he answers YES.

Thank you, I am here all week ;)

edited Dec 15 '18 at 8:57

ManyPinkHats

6,17922847

answered Dec 30 '14 at 3:45

user3035350

17913

add a comment |

My answer, assuming I ask one question only...

Is the liar in front of the death door (DD)?

A. (if the Truth guard is asked) If the Truth guy is in front of DD, he answers NO. If not, he answers YES.

B. (if the Liar guard is asked) If the Liar is in front of DD, he answers NO. If not, he answers YES.

Thank you, I am here all week ;)

edited Dec 15 '18 at 8:57

ManyPinkHats

6,17922847

answered Dec 30 '14 at 3:45

user3035350

17913

add a comment |

My answer, assuming I ask one question only...

Is the liar in front of the death door (DD)?

A. (if the Truth guard is asked) If the Truth guy is in front of DD, he answers NO. If not, he answers YES.

B. (if the Liar guard is asked) If the Liar is in front of DD, he answers NO. If not, he answers YES.

Thank you, I am here all week ;)

edited Dec 15 '18 at 8:57

ManyPinkHats

6,17922847

answered Dec 30 '14 at 3:45

user3035350

17913

My answer, assuming I ask one question only...

Is the liar in front of the death door (DD)?

A. (if the Truth guard is asked) If the Truth guy is in front of DD, he answers NO. If not, he answers YES.

B. (if the Liar guard is asked) If the Liar is in front of DD, he answers NO. If not, he answers YES.

Thank you, I am here all week ;)

edited Dec 15 '18 at 8:57

ManyPinkHats

6,17922847

answered Dec 30 '14 at 3:45

user3035350

17913

edited Dec 15 '18 at 8:57

ManyPinkHats

6,17922847

edited Dec 15 '18 at 8:57

ManyPinkHats

6,17922847

edited Dec 15 '18 at 8:57

ManyPinkHats

6,17922847

answered Dec 30 '14 at 3:45

user3035350

17913

answered Dec 30 '14 at 3:45

user3035350

17913

answered Dec 30 '14 at 3:45

user3035350

17913

add a comment |

EDIT: As @Ben mentioned, this answer is not really matching the logic-puzzle tag. Sorry.

If the guards are...

~~stupid~~ polite

you can

ask one to open a door

so you

didn't open a door (yet)

but

you see what's behind the door the guard opened

and

you can choose the right one (freedom, probably).

This doesn't even use the "truth-telling, lying" fact.

This only works if

The guards do what you say.

You can see freedom / the hangman when the door is open.

edited Jun 14 '16 at 15:39

answered Jun 14 '16 at 15:32

palsch

616715

3

$begingroup$
The OP set a tag called logic-puzzle. This is not a logical answer as you are locked up and they are guarding you from escaping. Please respect the question. (Sorry for being a bit harsh :-)). DISCLAIMER - THIS IS A SUBJECTIVE OPINION OF MINE
$endgroup$
– Ben
Jun 14 '16 at 15:36

$begingroup$
@Ben I'm sorry, I just thing lateral. lateral-thinking anywhere. Perhaps this is a problem of mine.
$endgroup$
– palsch
Jun 14 '16 at 15:38

$begingroup$
fair enough :-) no worries
$endgroup$
– Ben
Jun 14 '16 at 15:39

$begingroup$
Interesting idea, but how would you ask them? You: "Could you please open door 1?" Guard: Yes. (And stays standing there because while he can open the door, he won't.)
$endgroup$
– user1361991
Jun 14 '16 at 16:31

3

$begingroup$
@palsch Hmmm along that line of thinking: "Would you be a good chap and fetch the hangman for me?" should get the guard off in the right direction ;).
$endgroup$
– user1361991
Jun 14 '16 at 17:43

|
show 2 more comments

EDIT: As @Ben mentioned, this answer is not really matching the logic-puzzle tag. Sorry.

If the guards are...

~~stupid~~ polite

you can

ask one to open a door

so you

didn't open a door (yet)

but

you see what's behind the door the guard opened

and

you can choose the right one (freedom, probably).

This doesn't even use the "truth-telling, lying" fact.

This only works if

The guards do what you say.

You can see freedom / the hangman when the door is open.

edited Jun 14 '16 at 15:39

answered Jun 14 '16 at 15:32

palsch

616715

3

$begingroup$
The OP set a tag called logic-puzzle. This is not a logical answer as you are locked up and they are guarding you from escaping. Please respect the question. (Sorry for being a bit harsh :-)). DISCLAIMER - THIS IS A SUBJECTIVE OPINION OF MINE
$endgroup$
– Ben
Jun 14 '16 at 15:36

$begingroup$
@Ben I'm sorry, I just thing lateral. lateral-thinking anywhere. Perhaps this is a problem of mine.
$endgroup$
– palsch
Jun 14 '16 at 15:38

$begingroup$
fair enough :-) no worries
$endgroup$
– Ben
Jun 14 '16 at 15:39

$begingroup$
Interesting idea, but how would you ask them? You: "Could you please open door 1?" Guard: Yes. (And stays standing there because while he can open the door, he won't.)
$endgroup$
– user1361991
Jun 14 '16 at 16:31

3

$begingroup$
@palsch Hmmm along that line of thinking: "Would you be a good chap and fetch the hangman for me?" should get the guard off in the right direction ;).
$endgroup$
– user1361991
Jun 14 '16 at 17:43

|
show 2 more comments

EDIT: As @Ben mentioned, this answer is not really matching the logic-puzzle tag. Sorry.

If the guards are...

~~stupid~~ polite

you can

ask one to open a door

so you

didn't open a door (yet)

but

you see what's behind the door the guard opened

and

you can choose the right one (freedom, probably).

This doesn't even use the "truth-telling, lying" fact.

This only works if

The guards do what you say.

You can see freedom / the hangman when the door is open.

edited Jun 14 '16 at 15:39

answered Jun 14 '16 at 15:32

palsch

616715

EDIT: As @Ben mentioned, this answer is not really matching the logic-puzzle tag. Sorry.

If the guards are...

~~stupid~~ polite

you can

ask one to open a door

so you

didn't open a door (yet)

but

you see what's behind the door the guard opened

and

you can choose the right one (freedom, probably).

This doesn't even use the "truth-telling, lying" fact.

This only works if

The guards do what you say.

You can see freedom / the hangman when the door is open.

edited Jun 14 '16 at 15:39

answered Jun 14 '16 at 15:32

palsch

616715

edited Jun 14 '16 at 15:39

answered Jun 14 '16 at 15:32

palsch

616715

answered Jun 14 '16 at 15:32

palsch

616715

answered Jun 14 '16 at 15:32

palsch

616715

3

$begingroup$
The OP set a tag called logic-puzzle. This is not a logical answer as you are locked up and they are guarding you from escaping. Please respect the question. (Sorry for being a bit harsh :-)). DISCLAIMER - THIS IS A SUBJECTIVE OPINION OF MINE
$endgroup$
– Ben
Jun 14 '16 at 15:36

$begingroup$
@Ben I'm sorry, I just thing lateral. lateral-thinking anywhere. Perhaps this is a problem of mine.
$endgroup$
– palsch
Jun 14 '16 at 15:38

$begingroup$
fair enough :-) no worries
$endgroup$
– Ben
Jun 14 '16 at 15:39

$begingroup$
Interesting idea, but how would you ask them? You: "Could you please open door 1?" Guard: Yes. (And stays standing there because while he can open the door, he won't.)
$endgroup$
– user1361991
Jun 14 '16 at 16:31

3

$begingroup$
@palsch Hmmm along that line of thinking: "Would you be a good chap and fetch the hangman for me?" should get the guard off in the right direction ;).
$endgroup$
– user1361991
Jun 14 '16 at 17:43

|
show 2 more comments

3

$begingroup$
The OP set a tag called logic-puzzle. This is not a logical answer as you are locked up and they are guarding you from escaping. Please respect the question. (Sorry for being a bit harsh :-)). DISCLAIMER - THIS IS A SUBJECTIVE OPINION OF MINE
$endgroup$
– Ben
Jun 14 '16 at 15:36

$begingroup$
@Ben I'm sorry, I just thing lateral. lateral-thinking anywhere. Perhaps this is a problem of mine.
$endgroup$
– palsch
Jun 14 '16 at 15:38

$begingroup$
fair enough :-) no worries
$endgroup$
– Ben
Jun 14 '16 at 15:39

$begingroup$
Interesting idea, but how would you ask them? You: "Could you please open door 1?" Guard: Yes. (And stays standing there because while he can open the door, he won't.)
$endgroup$
– user1361991
Jun 14 '16 at 16:31

3

$begingroup$
@palsch Hmmm along that line of thinking: "Would you be a good chap and fetch the hangman for me?" should get the guard off in the right direction ;).
$endgroup$
– user1361991
Jun 14 '16 at 17:43

The OP set a tag called logic-puzzle. This is not a logical answer as you are locked up and they are guarding you from escaping. Please respect the question. (Sorry for being a bit harsh :-)). DISCLAIMER - THIS IS A SUBJECTIVE OPINION OF MINE

– Ben
Jun 14 '16 at 15:36

@Ben I'm sorry, I just thing lateral. lateral-thinking anywhere. Perhaps this is a problem of mine.

– palsch
Jun 14 '16 at 15:38

fair enough :-) no worries

– Ben
Jun 14 '16 at 15:39

Interesting idea, but how would you ask them? You: "Could you please open door 1?" Guard: Yes. (And stays standing there because while he can open the door, he won't.)

– user1361991
Jun 14 '16 at 16:31

@palsch Hmmm along that line of thinking: "Would you be a good chap and fetch the hangman for me?" should get the guard off in the right direction ;).

– user1361991
Jun 14 '16 at 17:43

|
show 2 more comments

You ask:

Would the other guard say your door is the door to freedom?

If either guard is guarding the door to freedom both would answer NO.
If either guard is guarding the door to death both would answer YES.
NO leads to freedom and YES leads to death.

I don't think there is one question whose answer would indicate which guard
is guarding which door and which door leads to freedom.

edited Sep 14 '18 at 4:16

Yout Ried

1,244624

answered Jan 25 '15 at 17:51

Jim-EIL

5113

add a comment |

You ask:

Would the other guard say your door is the door to freedom?

If either guard is guarding the door to freedom both would answer NO.
If either guard is guarding the door to death both would answer YES.
NO leads to freedom and YES leads to death.

I don't think there is one question whose answer would indicate which guard
is guarding which door and which door leads to freedom.

edited Sep 14 '18 at 4:16

Yout Ried

1,244624

answered Jan 25 '15 at 17:51

Jim-EIL

5113

add a comment |

You ask:

Would the other guard say your door is the door to freedom?

If either guard is guarding the door to freedom both would answer NO.
If either guard is guarding the door to death both would answer YES.
NO leads to freedom and YES leads to death.

I don't think there is one question whose answer would indicate which guard
is guarding which door and which door leads to freedom.

edited Sep 14 '18 at 4:16

Yout Ried

1,244624

answered Jan 25 '15 at 17:51

Jim-EIL

5113

You ask:

Would the other guard say your door is the door to freedom?

If either guard is guarding the door to freedom both would answer NO.
If either guard is guarding the door to death both would answer YES.
NO leads to freedom and YES leads to death.

I don't think there is one question whose answer would indicate which guard
is guarding which door and which door leads to freedom.

edited Sep 14 '18 at 4:16

Yout Ried

1,244624

answered Jan 25 '15 at 17:51

Jim-EIL

5113

edited Sep 14 '18 at 4:16

Yout Ried

1,244624

edited Sep 14 '18 at 4:16

Yout Ried

1,244624

edited Sep 14 '18 at 4:16

Yout Ried

1,244624

answered Jan 25 '15 at 17:51

Jim-EIL

5113

answered Jan 25 '15 at 17:51

Jim-EIL

5113

answered Jan 25 '15 at 17:51

Jim-EIL

5113

add a comment |

I am a newbie to Stack-Exchange and if my answer violates some guidelines , please correct me .

Let us assume , WLOG that the right door leads to freedom . So , the question I would ask is :

If I were to ask you whether the right door leads to freedom , would you answer yes?

The honest person would answer YES

The liar's internal response would be to say NO but being the liar he is , he would

say YES (due to double negation)

I think this would help you ascertain as to which is the truth-teller and which is the liar

edited May 23 '16 at 14:09

manshu

5,2581553

answered Dec 24 '14 at 5:04

pranav

5672720

2

$begingroup$
This answer is identical to (or at best a rehash of) Kevin's answer.
$endgroup$
– March Ho
Dec 30 '14 at 11:34

$begingroup$
@MarchHo Well, some people tend to not check whether their answer is already there, in order to avoid spoilers. Chances are both had the same idea independently that way.
$endgroup$
– Egor Hans
Sep 15 '18 at 20:32

$begingroup$
Would, "If I were to ask you if the wrong door leads to the hangman, would you answer no?", also do?
$endgroup$
– Martin Rattigan
Feb 27 at 23:57

add a comment |

I am a newbie to Stack-Exchange and if my answer violates some guidelines , please correct me .

Let us assume , WLOG that the right door leads to freedom . So , the question I would ask is :

If I were to ask you whether the right door leads to freedom , would you answer yes?

The honest person would answer YES

The liar's internal response would be to say NO but being the liar he is , he would

say YES (due to double negation)

I think this would help you ascertain as to which is the truth-teller and which is the liar

edited May 23 '16 at 14:09

manshu

5,2581553

answered Dec 24 '14 at 5:04

pranav

5672720

2

$begingroup$
This answer is identical to (or at best a rehash of) Kevin's answer.
$endgroup$
– March Ho
Dec 30 '14 at 11:34

$begingroup$
@MarchHo Well, some people tend to not check whether their answer is already there, in order to avoid spoilers. Chances are both had the same idea independently that way.
$endgroup$
– Egor Hans
Sep 15 '18 at 20:32

$begingroup$
Would, "If I were to ask you if the wrong door leads to the hangman, would you answer no?", also do?
$endgroup$
– Martin Rattigan
Feb 27 at 23:57

add a comment |

I am a newbie to Stack-Exchange and if my answer violates some guidelines , please correct me .

Let us assume , WLOG that the right door leads to freedom . So , the question I would ask is :

If I were to ask you whether the right door leads to freedom , would you answer yes?

The honest person would answer YES

The liar's internal response would be to say NO but being the liar he is , he would

say YES (due to double negation)

I think this would help you ascertain as to which is the truth-teller and which is the liar

edited May 23 '16 at 14:09

manshu

5,2581553

answered Dec 24 '14 at 5:04

pranav

5672720

I am a newbie to Stack-Exchange and if my answer violates some guidelines , please correct me .

Let us assume , WLOG that the right door leads to freedom . So , the question I would ask is :

If I were to ask you whether the right door leads to freedom , would you answer yes?

The honest person would answer YES

The liar's internal response would be to say NO but being the liar he is , he would

say YES (due to double negation)

I think this would help you ascertain as to which is the truth-teller and which is the liar

edited May 23 '16 at 14:09

manshu

5,2581553

answered Dec 24 '14 at 5:04

pranav

5672720

edited May 23 '16 at 14:09

manshu

5,2581553

edited May 23 '16 at 14:09

manshu

5,2581553

edited May 23 '16 at 14:09

manshu

5,2581553

answered Dec 24 '14 at 5:04

pranav

5672720

answered Dec 24 '14 at 5:04

pranav

5672720

answered Dec 24 '14 at 5:04

pranav

5672720

2

$begingroup$
This answer is identical to (or at best a rehash of) Kevin's answer.
$endgroup$
– March Ho
Dec 30 '14 at 11:34

$begingroup$
@MarchHo Well, some people tend to not check whether their answer is already there, in order to avoid spoilers. Chances are both had the same idea independently that way.
$endgroup$
– Egor Hans
Sep 15 '18 at 20:32

$begingroup$
Would, "If I were to ask you if the wrong door leads to the hangman, would you answer no?", also do?
$endgroup$
– Martin Rattigan
Feb 27 at 23:57

add a comment |

2

$begingroup$
This answer is identical to (or at best a rehash of) Kevin's answer.
$endgroup$
– March Ho
Dec 30 '14 at 11:34

$begingroup$
@MarchHo Well, some people tend to not check whether their answer is already there, in order to avoid spoilers. Chances are both had the same idea independently that way.
$endgroup$
– Egor Hans
Sep 15 '18 at 20:32

$begingroup$
Would, "If I were to ask you if the wrong door leads to the hangman, would you answer no?", also do?
$endgroup$
– Martin Rattigan
Feb 27 at 23:57

This answer is identical to (or at best a rehash of) Kevin's answer.

– March Ho
Dec 30 '14 at 11:34

@MarchHo Well, some people tend to not check whether their answer is already there, in order to avoid spoilers. Chances are both had the same idea independently that way.

– Egor Hans
Sep 15 '18 at 20:32

Would, "If I were to ask you if the wrong door leads to the hangman, would you answer no?", also do?

– Martin Rattigan
Feb 27 at 23:57

add a comment |

I'd like to add an explanation for the answer of Florian F:

If we look at it from the other side, the possibilities are the following:

The door is correct and the guard tells the truth: Then 1 is false, 3 is true, 2 depends on how the guide answers the whole (true if no, false if yes), which fits in both cases (011= no, 001 = yes) => the guard can answer in any way and will pick one answer.

The door is correct and the guard lies: Then 1 is true, 3 is true, 2 is same as above, which in both cases is a lie (111=no=>yes, 101=yes=>no) => the guard can answer in any way and will pick one answer.

The door is not correct and the guard tells the truth: Then 1 is false, 3 is false, 2 is same again, but this time it does not fit (010=yes >< 2 true, 000=no >< 2 false) => the guard cannot answer.

The door is not correct and the guard lies: Then 1 is true, 3 is false, 2 is same again, but this time it's the truth (110=no=>yes -- 2 false, 100=yes=>no -- 2 true) => the guard cannot answer.

Thus, the reasoning is: If the guard can answer then you pointed at the freedom door, if it cannot answer you pointed at the death door.

It was at this moment that Florian knew: He fucked up.

answered Jun 14 '16 at 15:13

Egor Hans

756

$begingroup$
If, however, there's a way to rather safely tell the guard cannot answer, then you're fine.
$endgroup$
– Egor Hans
Sep 15 '18 at 20:54

add a comment |

I'd like to add an explanation for the answer of Florian F:

If we look at it from the other side, the possibilities are the following:

The door is correct and the guard tells the truth: Then 1 is false, 3 is true, 2 depends on how the guide answers the whole (true if no, false if yes), which fits in both cases (011= no, 001 = yes) => the guard can answer in any way and will pick one answer.

The door is correct and the guard lies: Then 1 is true, 3 is true, 2 is same as above, which in both cases is a lie (111=no=>yes, 101=yes=>no) => the guard can answer in any way and will pick one answer.

The door is not correct and the guard tells the truth: Then 1 is false, 3 is false, 2 is same again, but this time it does not fit (010=yes >< 2 true, 000=no >< 2 false) => the guard cannot answer.

The door is not correct and the guard lies: Then 1 is true, 3 is false, 2 is same again, but this time it's the truth (110=no=>yes -- 2 false, 100=yes=>no -- 2 true) => the guard cannot answer.

Thus, the reasoning is: If the guard can answer then you pointed at the freedom door, if it cannot answer you pointed at the death door.

It was at this moment that Florian knew: He fucked up.

answered Jun 14 '16 at 15:13

Egor Hans

756

$begingroup$
If, however, there's a way to rather safely tell the guard cannot answer, then you're fine.
$endgroup$
– Egor Hans
Sep 15 '18 at 20:54

add a comment |

I'd like to add an explanation for the answer of Florian F:

If we look at it from the other side, the possibilities are the following:

The door is correct and the guard tells the truth: Then 1 is false, 3 is true, 2 depends on how the guide answers the whole (true if no, false if yes), which fits in both cases (011= no, 001 = yes) => the guard can answer in any way and will pick one answer.

The door is correct and the guard lies: Then 1 is true, 3 is true, 2 is same as above, which in both cases is a lie (111=no=>yes, 101=yes=>no) => the guard can answer in any way and will pick one answer.

The door is not correct and the guard tells the truth: Then 1 is false, 3 is false, 2 is same again, but this time it does not fit (010=yes >< 2 true, 000=no >< 2 false) => the guard cannot answer.

The door is not correct and the guard lies: Then 1 is true, 3 is false, 2 is same again, but this time it's the truth (110=no=>yes -- 2 false, 100=yes=>no -- 2 true) => the guard cannot answer.

Thus, the reasoning is: If the guard can answer then you pointed at the freedom door, if it cannot answer you pointed at the death door.

It was at this moment that Florian knew: He fucked up.

answered Jun 14 '16 at 15:13

Egor Hans

756

I'd like to add an explanation for the answer of Florian F:

If we look at it from the other side, the possibilities are the following:

The door is correct and the guard tells the truth: Then 1 is false, 3 is true, 2 depends on how the guide answers the whole (true if no, false if yes), which fits in both cases (011= no, 001 = yes) => the guard can answer in any way and will pick one answer.

The door is correct and the guard lies: Then 1 is true, 3 is true, 2 is same as above, which in both cases is a lie (111=no=>yes, 101=yes=>no) => the guard can answer in any way and will pick one answer.

The door is not correct and the guard tells the truth: Then 1 is false, 3 is false, 2 is same again, but this time it does not fit (010=yes >< 2 true, 000=no >< 2 false) => the guard cannot answer.

The door is not correct and the guard lies: Then 1 is true, 3 is false, 2 is same again, but this time it's the truth (110=no=>yes -- 2 false, 100=yes=>no -- 2 true) => the guard cannot answer.

Thus, the reasoning is: If the guard can answer then you pointed at the freedom door, if it cannot answer you pointed at the death door.

It was at this moment that Florian knew: He fucked up.

answered Jun 14 '16 at 15:13

Egor Hans

756

answered Jun 14 '16 at 15:13

Egor Hans

756

answered Jun 14 '16 at 15:13

Egor Hans

756

answered Jun 14 '16 at 15:13

Egor Hans

756

$begingroup$
If, however, there's a way to rather safely tell the guard cannot answer, then you're fine.
$endgroup$
– Egor Hans
Sep 15 '18 at 20:54

add a comment |

$begingroup$
If, however, there's a way to rather safely tell the guard cannot answer, then you're fine.
$endgroup$
– Egor Hans
Sep 15 '18 at 20:54

If, however, there's a way to rather safely tell the guard cannot answer, then you're fine.

– Egor Hans
Sep 15 '18 at 20:54

add a comment |

answered Jan 23 '15 at 22:37

Matt O'Brien

391

add a comment |

answered Jan 23 '15 at 22:37

Matt O'Brien

391

add a comment |

answered Jan 23 '15 at 22:37

Matt O'Brien

391

answered Jan 23 '15 at 22:37

Matt O'Brien

391

answered Jan 23 '15 at 22:37

Matt O'Brien

391

answered Jan 23 '15 at 22:37

Matt O'Brien

391

answered Jan 23 '15 at 22:37

Matt O'Brien

391

add a comment |

As with all such questions you could try:

If the question Q (in this case, "Is this the dodgy door") were to be answered with the same truth or falsehood as you are about to answer this question, would the answer be "yes" (or maybe "pish" if you can remember that this is a native word for "yes" or "no" without exactly recalling which)?

edited Feb 8 '17 at 5:02

answered Feb 8 '17 at 4:53

Martin Rattigan

1739

$begingroup$
This solution is actually surprisingly hard to analyze..
$endgroup$
– Egor Hans
Sep 15 '18 at 20:28

add a comment |

As with all such questions you could try:

If the question Q (in this case, "Is this the dodgy door") were to be answered with the same truth or falsehood as you are about to answer this question, would the answer be "yes" (or maybe "pish" if you can remember that this is a native word for "yes" or "no" without exactly recalling which)?

edited Feb 8 '17 at 5:02

answered Feb 8 '17 at 4:53

Martin Rattigan

1739

$begingroup$
This solution is actually surprisingly hard to analyze..
$endgroup$
– Egor Hans
Sep 15 '18 at 20:28

add a comment |

As with all such questions you could try:

If the question Q (in this case, "Is this the dodgy door") were to be answered with the same truth or falsehood as you are about to answer this question, would the answer be "yes" (or maybe "pish" if you can remember that this is a native word for "yes" or "no" without exactly recalling which)?

edited Feb 8 '17 at 5:02

answered Feb 8 '17 at 4:53

Martin Rattigan

1739

As with all such questions you could try:

If the question Q (in this case, "Is this the dodgy door") were to be answered with the same truth or falsehood as you are about to answer this question, would the answer be "yes" (or maybe "pish" if you can remember that this is a native word for "yes" or "no" without exactly recalling which)?

edited Feb 8 '17 at 5:02

answered Feb 8 '17 at 4:53

Martin Rattigan

1739

edited Feb 8 '17 at 5:02

answered Feb 8 '17 at 4:53

Martin Rattigan

1739

answered Feb 8 '17 at 4:53

Martin Rattigan

1739

answered Feb 8 '17 at 4:53

Martin Rattigan

1739

$begingroup$
This solution is actually surprisingly hard to analyze..
$endgroup$
– Egor Hans
Sep 15 '18 at 20:28

add a comment |

$begingroup$
This solution is actually surprisingly hard to analyze..
$endgroup$
– Egor Hans
Sep 15 '18 at 20:28

This solution is actually surprisingly hard to analyze..

– Egor Hans
Sep 15 '18 at 20:28

add a comment |

Update: My answer is wrong.

If anyone gets confused like I did, please see the comments for a reminder.

(original)

All of the answers are incorrect, and the puzzle itself flawed.

To reiterate the (overlooked) requirements:

"... you can only ask a single question from one of the
guards", and;

"One of the guards always tells the truth and the
other always lies. You don't know which one is [which]."

These rules create a situation where there is no method for discerning one guards' honesty from the other, and thus which door leads to freedom.

It's best explained as, "take my word for it*.

Edit - For Example, you ...

Ask Guard-A : "Do you guard the door to Freedom?" |OR| "Does Guard-B guard the door to freedom?"

Guard-A replies: "Yes"

50% - Guard-A is lying, you die.
50% - Guard-A is honest, you live.

edited Nov 14 '14 at 15:22

answered Nov 14 '14 at 10:10

Koffy

1173

3

$begingroup$
You don't have to discern the guards - you have to discern the doors. If you try all possible situations with the top two answers, you'll see you'll always pick the right door. Both ask one question from one guard who may or may not be the truthteller.
$endgroup$
– EagleV_Attnam
Nov 14 '14 at 10:26

$begingroup$
The answers are misleading because they immediately state "The honest-guard .. blah". How can you possible know which guard is which without asking both. See? Otherwise, just ask the honest guard and be done with the puzzle...
$endgroup$
– Koffy
Nov 14 '14 at 11:52

5

$begingroup$
@Koffy no, they start "If you asked the truth-guard," and go on to say "If you asked the liar-guard," thus demonstrating that you can safely act on the answer no matter which guard you happen to ask.
$endgroup$
– Kate Gregory
Nov 14 '14 at 13:09

2

$begingroup$
I can see my error now, thanks. I was never properly separating the 'defined behavior' from 'perceived behavior'. All in my head, the liar-guard would break behavior to .. achieve confusion. I even drew it on paper! I won't change my answer, but will add a note :) ty
$endgroup$
– Koffy
Nov 14 '14 at 15:16

add a comment |

Update: My answer is wrong.

If anyone gets confused like I did, please see the comments for a reminder.

(original)

All of the answers are incorrect, and the puzzle itself flawed.

To reiterate the (overlooked) requirements:

"... you can only ask a single question from one of the
guards", and;

"One of the guards always tells the truth and the
other always lies. You don't know which one is [which]."

These rules create a situation where there is no method for discerning one guards' honesty from the other, and thus which door leads to freedom.

It's best explained as, "take my word for it*.

Edit - For Example, you ...

Ask Guard-A : "Do you guard the door to Freedom?" |OR| "Does Guard-B guard the door to freedom?"

Guard-A replies: "Yes"

50% - Guard-A is lying, you die.
50% - Guard-A is honest, you live.

edited Nov 14 '14 at 15:22

answered Nov 14 '14 at 10:10

Koffy

1173

3

$begingroup$
You don't have to discern the guards - you have to discern the doors. If you try all possible situations with the top two answers, you'll see you'll always pick the right door. Both ask one question from one guard who may or may not be the truthteller.
$endgroup$
– EagleV_Attnam
Nov 14 '14 at 10:26

$begingroup$
The answers are misleading because they immediately state "The honest-guard .. blah". How can you possible know which guard is which without asking both. See? Otherwise, just ask the honest guard and be done with the puzzle...
$endgroup$
– Koffy
Nov 14 '14 at 11:52

5

$begingroup$
@Koffy no, they start "If you asked the truth-guard," and go on to say "If you asked the liar-guard," thus demonstrating that you can safely act on the answer no matter which guard you happen to ask.
$endgroup$
– Kate Gregory
Nov 14 '14 at 13:09

2

$begingroup$
I can see my error now, thanks. I was never properly separating the 'defined behavior' from 'perceived behavior'. All in my head, the liar-guard would break behavior to .. achieve confusion. I even drew it on paper! I won't change my answer, but will add a note :) ty
$endgroup$
– Koffy
Nov 14 '14 at 15:16

add a comment |

Update: My answer is wrong.

If anyone gets confused like I did, please see the comments for a reminder.

(original)

All of the answers are incorrect, and the puzzle itself flawed.

To reiterate the (overlooked) requirements:

"... you can only ask a single question from one of the
guards", and;

"One of the guards always tells the truth and the
other always lies. You don't know which one is [which]."

These rules create a situation where there is no method for discerning one guards' honesty from the other, and thus which door leads to freedom.

It's best explained as, "take my word for it*.

Edit - For Example, you ...

Ask Guard-A : "Do you guard the door to Freedom?" |OR| "Does Guard-B guard the door to freedom?"

Guard-A replies: "Yes"

50% - Guard-A is lying, you die.
50% - Guard-A is honest, you live.

edited Nov 14 '14 at 15:22

answered Nov 14 '14 at 10:10

Koffy

1173

Update: My answer is wrong.

If anyone gets confused like I did, please see the comments for a reminder.

(original)

All of the answers are incorrect, and the puzzle itself flawed.

To reiterate the (overlooked) requirements:

"... you can only ask a single question from one of the
guards", and;

"One of the guards always tells the truth and the
other always lies. You don't know which one is [which]."

These rules create a situation where there is no method for discerning one guards' honesty from the other, and thus which door leads to freedom.

It's best explained as, "take my word for it*.

Edit - For Example, you ...

Ask Guard-A : "Do you guard the door to Freedom?" |OR| "Does Guard-B guard the door to freedom?"

Guard-A replies: "Yes"

50% - Guard-A is lying, you die.
50% - Guard-A is honest, you live.

edited Nov 14 '14 at 15:22

answered Nov 14 '14 at 10:10

Koffy

1173

edited Nov 14 '14 at 15:22

answered Nov 14 '14 at 10:10

Koffy

1173

answered Nov 14 '14 at 10:10

Koffy

1173

answered Nov 14 '14 at 10:10

Koffy

1173

3

$begingroup$
You don't have to discern the guards - you have to discern the doors. If you try all possible situations with the top two answers, you'll see you'll always pick the right door. Both ask one question from one guard who may or may not be the truthteller.
$endgroup$
– EagleV_Attnam
Nov 14 '14 at 10:26

$begingroup$
The answers are misleading because they immediately state "The honest-guard .. blah". How can you possible know which guard is which without asking both. See? Otherwise, just ask the honest guard and be done with the puzzle...
$endgroup$
– Koffy
Nov 14 '14 at 11:52

5

$begingroup$
@Koffy no, they start "If you asked the truth-guard," and go on to say "If you asked the liar-guard," thus demonstrating that you can safely act on the answer no matter which guard you happen to ask.
$endgroup$
– Kate Gregory
Nov 14 '14 at 13:09

2

$begingroup$
I can see my error now, thanks. I was never properly separating the 'defined behavior' from 'perceived behavior'. All in my head, the liar-guard would break behavior to .. achieve confusion. I even drew it on paper! I won't change my answer, but will add a note :) ty
$endgroup$
– Koffy
Nov 14 '14 at 15:16

add a comment |

3

$begingroup$
You don't have to discern the guards - you have to discern the doors. If you try all possible situations with the top two answers, you'll see you'll always pick the right door. Both ask one question from one guard who may or may not be the truthteller.
$endgroup$
– EagleV_Attnam
Nov 14 '14 at 10:26

$begingroup$
The answers are misleading because they immediately state "The honest-guard .. blah". How can you possible know which guard is which without asking both. See? Otherwise, just ask the honest guard and be done with the puzzle...
$endgroup$
– Koffy
Nov 14 '14 at 11:52

5

$begingroup$
@Koffy no, they start "If you asked the truth-guard," and go on to say "If you asked the liar-guard," thus demonstrating that you can safely act on the answer no matter which guard you happen to ask.
$endgroup$
– Kate Gregory
Nov 14 '14 at 13:09

2

$begingroup$
I can see my error now, thanks. I was never properly separating the 'defined behavior' from 'perceived behavior'. All in my head, the liar-guard would break behavior to .. achieve confusion. I even drew it on paper! I won't change my answer, but will add a note :) ty
$endgroup$
– Koffy
Nov 14 '14 at 15:16

You don't have to discern the guards - you have to discern the doors. If you try all possible situations with the top two answers, you'll see you'll always pick the right door. Both ask one question from one guard who may or may not be the truthteller.

– EagleV_Attnam
Nov 14 '14 at 10:26

The answers are misleading because they immediately state "The honest-guard .. blah". How can you possible know which guard is which without asking both. See? Otherwise, just ask the honest guard and be done with the puzzle...

– Koffy
Nov 14 '14 at 11:52

@Koffy no, they start "If you asked the truth-guard," and go on to say "If you asked the liar-guard," thus demonstrating that you can safely act on the answer no matter which guard you happen to ask.

– Kate Gregory
Nov 14 '14 at 13:09

I can see my error now, thanks. I was never properly separating the 'defined behavior' from 'perceived behavior'. All in my head, the liar-guard would break behavior to .. achieve confusion. I even drew it on paper! I won't change my answer, but will add a note :) ty

– Koffy
Nov 14 '14 at 15:16

add a comment |

Difference in color is mere an assumption,some other difference in gates could also be used in the question.

answered Jul 22 '16 at 12:45

Ayush Srivastava

651

1

$begingroup$
what if the guard doesn't say anything about correctness of your question, and answers like this: "the door that you are pointing will lead you to freedom"?
$endgroup$
– Rafe
Jul 23 '16 at 6:18

add a comment |

Difference in color is mere an assumption,some other difference in gates could also be used in the question.

answered Jul 22 '16 at 12:45

Ayush Srivastava

651

1

$begingroup$
what if the guard doesn't say anything about correctness of your question, and answers like this: "the door that you are pointing will lead you to freedom"?
$endgroup$
– Rafe
Jul 23 '16 at 6:18

add a comment |

Difference in color is mere an assumption,some other difference in gates could also be used in the question.

answered Jul 22 '16 at 12:45

Ayush Srivastava

651

Difference in color is mere an assumption,some other difference in gates could also be used in the question.

answered Jul 22 '16 at 12:45

Ayush Srivastava

651

answered Jul 22 '16 at 12:45

Ayush Srivastava

651

answered Jul 22 '16 at 12:45

Ayush Srivastava

651

answered Jul 22 '16 at 12:45

Ayush Srivastava

651

1

$begingroup$
what if the guard doesn't say anything about correctness of your question, and answers like this: "the door that you are pointing will lead you to freedom"?
$endgroup$
– Rafe
Jul 23 '16 at 6:18

add a comment |

1

$begingroup$
what if the guard doesn't say anything about correctness of your question, and answers like this: "the door that you are pointing will lead you to freedom"?
$endgroup$
– Rafe
Jul 23 '16 at 6:18

what if the guard doesn't say anything about correctness of your question, and answers like this: "the door that you are pointing will lead you to freedom"?

– Rafe
Jul 23 '16 at 6:18

add a comment |

The mechanics are more or less the same in this answer but for some reason it took me a long time to convince myself it worked every time.

edited Jun 29 '17 at 15:23

answered Jun 29 '17 at 15:13

Alex Bochel

267

$begingroup$
Ask the lieing guard that question, s/he will say no if the truth telling guard is in front of the door to freedom, taking the opposite door means you die. However which door the guard was referring to?
$endgroup$
– Matt Stevens
Feb 10 '18 at 11:22

1

$begingroup$
@MattStevens The answer actually does make sense. What you probably overlooked is that if you ask the liar, you refer to the door he's not standing in front of. That means that it fits: First, let's look at the scenario that you ask the guy in front of the freedom door. If you ask the truth-teller, the door behind him is the freedom door, and he says yes. If you ask the liar, the door behind him is the freedom door, so the door behind the other one (the truth-teller!) is death, which the liar negates to yes. Similar for the guard at the death door, who always says no.
$endgroup$
– Egor Hans
Jun 26 '18 at 13:01

add a comment |

The mechanics are more or less the same in this answer but for some reason it took me a long time to convince myself it worked every time.

edited Jun 29 '17 at 15:23

answered Jun 29 '17 at 15:13

Alex Bochel

267

$begingroup$
Ask the lieing guard that question, s/he will say no if the truth telling guard is in front of the door to freedom, taking the opposite door means you die. However which door the guard was referring to?
$endgroup$
– Matt Stevens
Feb 10 '18 at 11:22

1

$begingroup$
@MattStevens The answer actually does make sense. What you probably overlooked is that if you ask the liar, you refer to the door he's not standing in front of. That means that it fits: First, let's look at the scenario that you ask the guy in front of the freedom door. If you ask the truth-teller, the door behind him is the freedom door, and he says yes. If you ask the liar, the door behind him is the freedom door, so the door behind the other one (the truth-teller!) is death, which the liar negates to yes. Similar for the guard at the death door, who always says no.
$endgroup$
– Egor Hans
Jun 26 '18 at 13:01

add a comment |

The mechanics are more or less the same in this answer but for some reason it took me a long time to convince myself it worked every time.

edited Jun 29 '17 at 15:23

answered Jun 29 '17 at 15:13

Alex Bochel

267

The mechanics are more or less the same in this answer but for some reason it took me a long time to convince myself it worked every time.

edited Jun 29 '17 at 15:23

answered Jun 29 '17 at 15:13

Alex Bochel

267

edited Jun 29 '17 at 15:23

answered Jun 29 '17 at 15:13

Alex Bochel

267

answered Jun 29 '17 at 15:13

Alex Bochel

267

answered Jun 29 '17 at 15:13

Alex Bochel

267

$begingroup$
Ask the lieing guard that question, s/he will say no if the truth telling guard is in front of the door to freedom, taking the opposite door means you die. However which door the guard was referring to?
$endgroup$
– Matt Stevens
Feb 10 '18 at 11:22

1

$begingroup$
@MattStevens The answer actually does make sense. What you probably overlooked is that if you ask the liar, you refer to the door he's not standing in front of. That means that it fits: First, let's look at the scenario that you ask the guy in front of the freedom door. If you ask the truth-teller, the door behind him is the freedom door, and he says yes. If you ask the liar, the door behind him is the freedom door, so the door behind the other one (the truth-teller!) is death, which the liar negates to yes. Similar for the guard at the death door, who always says no.
$endgroup$
– Egor Hans
Jun 26 '18 at 13:01

add a comment |

$begingroup$
Ask the lieing guard that question, s/he will say no if the truth telling guard is in front of the door to freedom, taking the opposite door means you die. However which door the guard was referring to?
$endgroup$
– Matt Stevens
Feb 10 '18 at 11:22

1

$begingroup$
@MattStevens The answer actually does make sense. What you probably overlooked is that if you ask the liar, you refer to the door he's not standing in front of. That means that it fits: First, let's look at the scenario that you ask the guy in front of the freedom door. If you ask the truth-teller, the door behind him is the freedom door, and he says yes. If you ask the liar, the door behind him is the freedom door, so the door behind the other one (the truth-teller!) is death, which the liar negates to yes. Similar for the guard at the death door, who always says no.
$endgroup$
– Egor Hans
Jun 26 '18 at 13:01

Ask the lieing guard that question, s/he will say no if the truth telling guard is in front of the door to freedom, taking the opposite door means you die. However which door the guard was referring to?

– Matt Stevens
Feb 10 '18 at 11:22

@MattStevens The answer actually does make sense. What you probably overlooked is that if you ask the liar, you refer to the door he's not standing in front of. That means that it fits: First, let's look at the scenario that you ask the guy in front of the freedom door. If you ask the truth-teller, the door behind him is the freedom door, and he says yes. If you ask the liar, the door behind him is the freedom door, so the door behind the other one (the truth-teller!) is death, which the liar negates to yes. Similar for the guard at the death door, who always says no.

– Egor Hans
Jun 26 '18 at 13:01

add a comment |

You could also ask, "Why are there screams coming from behind this door?".

answered 18 mins ago

Martin Rattigan

1739

$begingroup$
This does not provide an answer to the question. To critique or request clarification from an author, leave a comment below their post. - From Review
$endgroup$
– F1Krazy
55 secs ago

add a comment |

You could also ask, "Why are there screams coming from behind this door?".

answered 18 mins ago

Martin Rattigan

1739

$begingroup$
This does not provide an answer to the question. To critique or request clarification from an author, leave a comment below their post. - From Review
$endgroup$
– F1Krazy
55 secs ago

add a comment |

You could also ask, "Why are there screams coming from behind this door?".

answered 18 mins ago

Martin Rattigan

1739

You could also ask, "Why are there screams coming from behind this door?".

answered 18 mins ago

Martin Rattigan

1739

answered 18 mins ago

Martin Rattigan

1739

answered 18 mins ago

Martin Rattigan

1739

answered 18 mins ago

Martin Rattigan

1739

$begingroup$
This does not provide an answer to the question. To critique or request clarification from an author, leave a comment below their post. - From Review
$endgroup$
– F1Krazy
55 secs ago

add a comment |

$begingroup$
This does not provide an answer to the question. To critique or request clarification from an author, leave a comment below their post. - From Review
$endgroup$
– F1Krazy
55 secs ago

This does not provide an answer to the question. To critique or request clarification from an author, leave a comment below their post. - From Review

– F1Krazy
55 secs ago

add a comment |

-2

answered Mar 11 '15 at 14:29

VicFore

131

4

$begingroup$
Welcome to puzzling.SE. "Everything should be made as simple as possible, but not simpler." - Albert Einstein
$endgroup$
– ABcDexter
Jun 14 '16 at 15:34

add a comment |

-2

answered Mar 11 '15 at 14:29

VicFore

131

4

$begingroup$
Welcome to puzzling.SE. "Everything should be made as simple as possible, but not simpler." - Albert Einstein
$endgroup$
– ABcDexter
Jun 14 '16 at 15:34

add a comment |

-2

answered Mar 11 '15 at 14:29

VicFore

131

answered Mar 11 '15 at 14:29

VicFore

131

answered Mar 11 '15 at 14:29

VicFore

131

answered Mar 11 '15 at 14:29

VicFore

131

answered Mar 11 '15 at 14:29

VicFore

131

4

$begingroup$
Welcome to puzzling.SE. "Everything should be made as simple as possible, but not simpler." - Albert Einstein
$endgroup$
– ABcDexter
Jun 14 '16 at 15:34

add a comment |

4

$begingroup$
Welcome to puzzling.SE. "Everything should be made as simple as possible, but not simpler." - Albert Einstein
$endgroup$
– ABcDexter
Jun 14 '16 at 15:34

Welcome to puzzling.SE. "Everything should be made as simple as possible, but not simpler." - Albert Einstein

– ABcDexter
Jun 14 '16 at 15:34

add a comment |

protected by Community♦ Mar 14 '15 at 21:51

This page is only for reference, If you need detailed information, please check here

搜尋此網誌

Htydjtk