Question

A traveler came to the island in which a part of the inhabitants always tells the truth and the others always lie. What question should the traveller ask an inhabitant to figure out if he is a truthful person or a liar?

Answer 1

He should ask a question like: «If you were a truthful person, how would you answer a question “Are you a liar?”?». Those who always say the truth will answer “no” and the liar will always answer “yes”.