# Fork in the road problem - with a single person

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.

• Welcome to Puzzling! Just wondering, did you write this puzzle yourself, or is it from a book/test/somewhere else? Mar 13, 2020 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 ... Mar 13, 2020 at 17:21

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.