Which Door?

Logic Level 2

At the bottom of a tower are two guards: one always tells the truth, while the other always lies.

One guard guards door 1, and the other guards door 2. One of the two doors is safe, and the other is deadly.

You ask the guard for door 1, "Would the other guard say this door is safe or deadly?"
He says, "Safe."

Which door is safe, door 1 or door 2?

Door 1 Door 2 Neither--the truth and lies cancel out

TheBlockWizard_ 87 by TheBlockWizard_ 87 , 34, United Kingdom

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2 solutions

Fayez Shahzad
Jan 19, 2018

G1 = Guard for Door 1 G2 = Guard for Door 2

If G1 = truthful then G2 = liar. Now, if G2 would say = Safe, then Door 1 = deadly

If G1 = liar then G2 = truthful. Now, if G2 would say = Safe, then Door 1 = safe

but as you're asking G1 and he's a liar, then Door 1 = deadly!

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Stephen Mellor
Jan 12, 2018

By the statement "would the other guard", this removes the ambiguity from the degree of truthfulness. This statement makes it includes 1 opinion from each guard, meaning that there is one truth and one lie incorporated, meaning that the statement is a lie. This works for either way round that the guards are. As it is a lie, Door 2 is the safe one.

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