The logician Raymond Smullyan describes an island containing

The logician Raymond Smullyan describes an island containing two types of people: knights who always tell the truth and knaves who always lie. You visit the island and are approached by two natives, John and Bill, who speak to you. John says, “Of Bill and I, exactly one of us is a knight.” Bill says, “Only a knave would say that John is a knave.”

What are John and Bill?

Solution

No one can say that he is a knave.

John say one of us is a knight thats truth one is knight and one is knave

Bill says John is knave thats lie because if this is the truth then the above sentence can\'t be a truth and we know that one is knight and one is knave so

Bill speaks lie and Bill is knaves

John speaks truth and John is knights