Answer to Question #214758 in Discrete Mathematics for Lucy

Question

You visit an island where three triplet brothers named Lanister, Lewis and Tom, live. They are indistinguishable in appearance, but Lanister and Lewis, are knaves whereas Tom is a knight. One day you meet one of the three on the street and wish to know whether or not he is Lewis, because Lewis owes you money. You are allowed to ask him only one yes/no question (your question may not have more than three words!) What question would work? Provide logical reasoning for your answer.

Answer 1

You should ask him: "Are you Lanister?".

If he say "Yes", then he is Lewis. If he say "No", then he is not Lewis.

Indeed, there are 3 cases:

1. You meet Lanister. Since he is knave, he say "No".
2. You meet Lewis. Since he is knave, he say "Yes".
3. You meet Tom. Since he is knight, he say "No".