## 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.

39 Comments on "The Honest and Dishonest Guards"Shower scene | Paul Hassing's autobiography. says

January 6, 2017 @ 17:23

[…] She seems genuine, but I’m reminded of a childhood riddle. […]

Bryant says

May 2, 2017 @ 23:02

The answer is as simple as math. I would ask either guy to put the correct answer to (for example) 9×9 on the door that leads to freedom. If its the guy that always lies he would put the wrong answer on the wrong door because he always lies. If it’s the guy that tells the truth he would put the correct answer on the correct door.

Rookie says

May 14, 2018 @ 10:10

I’d ask the first guard something I know the answer too – ‘how many fingers am I holding up?’ ‘What is the colour of that door?’ etc. From that answer, I can figure out if he tells the truth or lies, and figure out the other one.

Then I ask the other one which door is the correct one, and choose the one depending on if they are a liar or not.

Outcome 1) The first guard lies, so I pick the door the other guard tells me

Outcome 2) The first guard tells the truth, so I pick the door that the other guard doesn’t tell me.

Missing The Point says

June 7, 2018 @ 15:12

I would just go out from the the door I came in. :-)

Zachary Hemmer. says

June 11, 2018 @ 22:27

I thought about this one for a while, and I finally found an answer.

just ask “If you were the other guard, what would you say if i asked you what the other guard tells?”

the lair will say truth.

the honest will say lair.

Lying guard logic and answer.

Knowing that the other guard is honest, he knows that the other guard would be honest and say that he was the lair, so he lies and says “I would say ‘he tells the truth’”

Honest guard logic and answer.

Knowing that the other guard lies, he knows that the other guard would say that he was the liar, because he always lies. So he answers truthfully, saying “I would say “he is the one who lies”

Problem Solved

Chuck Cochems says

June 13, 2018 @ 00:57

The puzzle, as stated, is invalid. It cannot be solved.

Here’s why.

The lying guard can lie by saying “i don’t know”. This is definitely a lie, since the lying guard does know which door is correct. In fact ANY “If I were to ask you” question directed at the lying guard can be answered falsely with “I don’t know”. We know it’s a lie as soon as we hear it. But we gain no information other then the fact we asked the liar.

if we ask one guard about the other guards answer, we don’t get our answer either way. The liar can answer ” i don’t know” which is a lie, because he does know what the truth teller will say. The truthteller must either answer “I don’t know” (because the liar has two options) or “he will say he doesn’t know” (making the reasonable assumption that the liar wants to get you killed)”. either way you are screwed and do not find out which door is correct.

Asking the truth teller about his own answer does reveal the correct door, but you only have a 50/50 shot of getting the truth teller. it’s no different from just asking the truth teller directly.

If the problem is stated this way it is solvable.

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 can only tell lies. Neither guard desires your death. 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?

That qualifier about the lying guard not actively trying to get you killed means that he won’t just say “i don’t know’ which is what a malicious lying guard would do, which makes the puzzle solvable.

Ezzy says

July 28, 2018 @ 03:52

I would ask either one; ‘if you are a liar, tell me me the exit door?’

Being an honest guard, he will still be pointing the exit door. The liar will lie to his lie (double negation) thus points to the exit door.

Captain Dave says

August 9, 2018 @ 04:25

Easy…. Kill one guard and ask the remaining one, “Is he alive?” *pointing at the dead one*

That guard will answer Yes/No depending on which he is (Liar or Truth Teller), then the follow up based on his answer would be, “Which Door is safe?”

Dan says

August 9, 2018 @ 09:04

Only one question Captain Dave!

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