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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

 

50 Comments on "The Honest and Dishonest Guards"


Two Questions says
August 20, 2019 @ 10:14

If I can ask two questions at both Guards then there they are:

Question 1: Can the Guard who Speaks the Truth Lie it’s Truth?
-The Honest Guard will say no while the other will say yes.
Question 2: Which door is the Exit?
-By knowing which one lies and such then you already know who’ll point to the Exit.


Quak says
September 20, 2019 @ 17:21

If You ask A guard If the other one is alive, If he says no, You have the liar. Ask him which door is safe, When he tells you, Go through the opposite side.


Mark says
November 29, 2019 @ 15:01

To ask the right (single) question you must focus on the answer, not who is lying or telling the truth. Once you get the answer you know which door leads to freedom and you go through it without knowing which guard is the liar. You don’t dare,

Ask either guard (or both at the same time) “Is the honest guard standing by the door to freedom?” Whoever says “yes” is standing by the freedom door. The other guard will say “no” or “I don’t know”, anything but “yes” so don’t go through that door.

Honest-Freedom door…Yes
Honest-Prison Door……No
Liar- Freedom door…Yes or something else, but not no
Liar- Prison Door…….No or something else, but not yes

Remember you don’t care who is lying when you phrase the single question this way. But if the liar obfuscates instead of giving a yes or no you can assume he is also the liar. If the liar says yes or no then you don’t know who the liar is but you don’t need to know.


RoRoRowyerBoat says
December 7, 2019 @ 18:01

Ask a question that you already know the true answer to: Is the Earth round? Does 1 + 1 = 2? Is my name (whatever your name is). The dishonest guard can only lie, so he will always give the incorrect answer to these type of questions. Et voila.


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