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The Honest and Dishonest Guards

You’re in a room with two doors. There’s a guard at each door. One door is the exit, but behind the other door is something that will kill you. You’re told that one guard always tells the truth and the other guard always lies. You don’t know which guard is which. You are allowed to ask one question to either of the guards to determine which door is the exit.

What question should you ask?

Ask either guard what door the other guard would say is the exit, then choose the opposite door.

If you ask the guard who always tells the truth, he knows the other guard would lie, so he’ll point you to the door leading to death. If you ask the guard who always lies, he knows the other guard would truthfully show you the exit, so he’ll lie and point you to the door leading to death.

An alternate solution is to ask a guard what they would answer if you were to ask them which door was the exit, then choose that door. The truthful guard will point to the correct exit, but the lying guard will too. Here’s why. If you asked him what door was the exit, he would normally lie and point to the death door, but you asked him what he would say if you asked what door was the exit, and in order to lie to that question, he will point you to the exit.

Posted in Brain Teasers

 

43 Comments on "The Honest and Dishonest Guards"


Aswathy says
April 6, 2015 @ 18:23

Ask them, ” what would the other person say is the right door?” Both would lie, so pick the other door. Simple!


V says
April 24, 2015 @ 19:03

But how would you know which guard told the truth and which one was lying if you could only ask one question to one guard?


Aravind says
July 15, 2015 @ 11:42

This is how:

Go to any of the guards.

Ask him this question :

“If I ask that guard which is the correct door what will he say?”

If it is the liar, he will lie. If it is the truth teller, he will be saying about the other person, which is liar.

So if you ask this question to any one of them you will get a wrong answer.

But there are only two doors, and one of them is wrong.

So after asking that question, go to the door which he is NOT saying about.


Aravind says
July 15, 2015 @ 11:45

Ask any one of the guards this question:

“If i ask the other guard which the right door is what will he say?”

Then he will show us a door. If we asked a liar, he will lie. If we ask truth teller, he will lie because he is talking about the other guard, who is a liar.

So go to the other door than which he points to.


Haley Dewings says
October 13, 2015 @ 22:21

Easy! Ask, ‘Do both of the doors lead to the exit?’
The liar would say yes, and it’s pretty obvious which door to go through after that. I know I got it right!


rjsavvy says
October 15, 2015 @ 03:10

Just ask “what colour are my eyes” the liar will lie and you’d know or be told the truth and you’d also know!! Simple even if it isn’t the “correct” answer ?


Seth says
November 16, 2015 @ 12:52

Some of the commenters arent getting the point; if I asked one of the gaurds what color my hat or eyes or anything was, id find out which gaurd lies and which tells the truth. BUT…. in doing so, you waste your ONE question.

For those having trouble understanding the answer, follow this logic:
When asked, “which door would the other guy tell me is correct” you will get the wrong answer from both gaurds, which leaves only the correct door.

How? If you ask the liar, he will lie, giving you the wrong answer. If you ask the truth teller, he will give you the liar’s answer. So they both give you the wrong door. Which leaves only the correct door


Tony says
December 22, 2015 @ 14:34

How? If you ask the liar, he will lie, giving you the wrong answer. If you ask the truth teller, he will give you the liar’s answer. So they both give you the wrong door. Which leaves only the correct door

The liar will lie telling the truth and the honest guard will lie because the is telling the truth about the liar which is the opposite so you are back where you started.


Dan says
December 22, 2015 @ 15:16

@Tony Did you read the explanation in the answer?


Bonnie says
February 13, 2016 @ 15:30

Here’s another, albeit more confusing, version of the answer:
Ask either guard, “What would you say if I asked you which is the correct door?”
If you have asked the truth teller, he will be completely honest. If you had just flat-out asked him which door is correct, he would have been honest, so he will tell you the correct door.
If you had flat out asked the liar which door is correct, of course he would have lied. However, he must lie about the fact that he would have lied when faced with the question above. As a result, he will tell you the correct door.
No matter which guard you ask, therefore, you will be told the correct door.


bob says
April 5, 2016 @ 20:50

ask either guard: is the guard to paradise a truth teller.
if hell’s guard is truth, he will say no since heaven’s guard needs to lie
if hell’s guard is lie, he will say no since heaven’s guard will tell the truth
if heaven’s guard is truth, he will say yes since heaven’s guard will tell the truth
if heaven’s guard lies, he will say yes since he lies that he tells the truth


Jake says
April 11, 2016 @ 23:39

You ask either guard what the other guard would tell you is the door leading to life and the door leading to death and pick the opposite door of which they said.

If you ask that to the truth teller, he will say the lie that the lie teller would say to you which will give you the door leading to death and then pick the opposite.

If you ask that to the lie teller he will say the opposite of what the truth teller would say which would give you the door leading to death and then pick the opposite.


Sabbatai says
May 3, 2016 @ 21:50

Rjsaavy, you’ve now used your one question. You cannot ask either guard any questions despite knowing which is the liar and which tells the truth. Now you have to choose the door on your own.

Haley Dewings… what? How would that help you. Lol… it’s “obvious which door to go through” based on what? You’d be taking a 50/50 chance and your question to the guards would not have helped at all.


Yeshua Alexander says
May 22, 2016 @ 04:35

I have a solution I haven’t seen posted on this forum or any other: ask the guard if a liar stands in front of the door to death. If he says yes take his door. If he says no take the other.

I could explain but its tedious and i think simulating all 4 possibilities for yourselves would be more reassuring.


Eddie says
May 28, 2016 @ 16:48

C potato


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