# 347 logic/smullyan/fork.two.men.p

## Description

This article is from the Puzzles FAQ,
by Chris Cole chris@questrel.questrel.com and Matthew Daly
mwdaly@pobox.com with numerous contributions by others.

# 347 logic/smullyan/fork.two.men.p

Two men stand at a fork in the road. One fork leads to Someplaceorother; the

other fork leads to Nowheresville. One of these people always answers the

truth to any yes/no question which is asked of him. The other always lies

when asked any yes/no question. By asking one yes/no question, can you

determine the road to Someplaceorother?

logic/smullyan/fork.two.men.s

The fact that there are two is a red herring - you only need one of

either type. You ask him the following question: "If I were to ask

you if the left fork leads to Someplaceorother, would you say 'yes'?"

If the person asked is a truthteller, he will answer "yes" if the left

fork leads to Someplaceorother, and "no" otherwise. But so will the

liar. So, either way, go left is the answer is "yes", and right otherwise.

It is possible, of course, that the liars are malicious, and they will tell

the truth if they figure out that you are trying to trick them.

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