Friday, April 16, 2010

Knights and Knaves

A very special island is inhabited only by knights and knaves. Knights always tell the truth, and knaves always lie. You meet four inhabitants: Sue, Alice, Homer and Ted. Sue tells you that Alice is a knave. Alice tells you that Ted is a knave. Homer claims, `Alice is a knight and Ted is a knave.' Ted tells you,... `I and Homer are different.'

So who is a knight and who is a knave?

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Assumption 1:

We assume Sue is a knave. This means Alice is a knight because the statement 'Alice is a knave' is definitely wrong. Therefore the statement 'Ted is a knave' is right because Alice is a knight. Then, Homer can also be assumed as a knight because the statement `Alice is a knight and Ted is a knave' is right. The problem is with the fourth statement. If Ted is a knave, the statement `I and Homer are different' is wrong, and the correct statement is `I and Homer are not different' or 'I and Homer are the same'. And this means, if Ted is a knave, Homer must also be a knave, thus the statement `Alice is a knight and Ted is a knave' is wrong, rendering the fist and second statement invalid. The conclusion is assumption 1 cannot be accepted.

Assumption 2:

We assume Sue is a knight. This means Alice is a knave because the statement 'Alice is a knave' is definitely right. Therefore the statement 'Ted is a knave' is wrong because Alice is a knave, thus Ted is a knight. Then, Homer can also be assumed as a knave because the statement `Alice is a knight and Ted is a knave' is wrong. The fourth statement, `I and Homer are different' is right if Ted is a knight, and it conforms to the situation if Ted is a knight dan Homer is a knave. The conclusion is assumption 2 can be accepted.

Therefore the answer is:

Sue and Ted are knights
Alice and Homer are knaves

Right?

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