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You walk to a fork from where two roads come out - one leading to a post office and the other to a university.

You want to go to the university, but do not know which road would lead you to it. A sentry standing on the fork knows the answer, but he has a habit of alternately speaking the truth and telling a lie.

What single question to the sentry will help you find the right road?

Stiv
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Nehal Samee
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  • Welcome to Puzzling! Just wondering, did you write this puzzle yourself, or is it from a book/test/somewhere else? – Rand al'Thor Mar 13 '20 at 17:13
  • @Randal'Thor Well....I was looking into some past questions of my varsity and found this. But I'm unaware of the fact that which professor made it ... – Nehal Samee Mar 13 '20 at 17:21

1 Answers1

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This is a variant of the classical two roads, two guards problem, except that

two different guards (one telling the truth, one lying) is replaced by two different statements by the same guard (one truth, one lie).

So the question should be:

If my next question will be "does this road lead to the post office", what will you say?

The sentry has a habit of alternately speaking the truth and telling a lie, so:

  • If his answer now is the truth, then

    his next statement will be a lie, so he will give you the false answer that he would give for the next question.

  • If his answer now is a lie, then

    his next statement will be truthful, so he will lie and give you the false answer now.

Either way, you've got your answer

about whether that road leads to the post office - which, since it's false, you take as the answer about whether it leads to the university.

Rand al'Thor
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