Question: The population of an island consists of two and only two types of people : the knights, who invariably tell the truth and the knaves who always lie. Cases 1, 2 and 3 are:
Case 1.There are only two inhabitants, X and Y. X says, "At least one of us is a knave." What are X and Y?
Case 2. Suppose X says, "Either I am a knave, or Y is a knight?" What are X and Y?
Case 3. Consider once more X, Y and Z each of who is either a knight or a knave. X says, "All of us are knaves." Y says, "Exactly one of us is a knight." What are X, Y and Z?
Answer:
Case 1 : X is Knight and Y is Knave.
Case 2 : Both are Knight.
Case 3: X and Z are Knaves, Y is Knight