A moderator takes a set of 8 stamps, 4 red and 4 green, known to three logicians, and affixes two to the forehead of each logician so that each logician can see all the other stamps except those two in the moderator's pocket and the two on his or her own head. He asks them in turn if they know the colors of their own stamps:
1. A: "No"
2. B: "No"
3. C: "No"
4. A: "No"
5. B: "Yes"

What are the colors of B's stamps?

B has one green and one red. A has two of the same color and C has two of opposite color A has. B knows that if B has two of the same color as C, A will know what A has immediately. B also knows that the same is true for C. Since neither A nor C knew what he respectively has on his own forehead, B knows that B has to have one of each. Statement 4 is a trick. B knew the answer after statement 3. A and C, if they were equal logicians to B, would know the answer to what they had on their respective foreheads as soon as B answered in statement 5.

(sorry for always using "he." it's easier than "he/she."

Posted by a.t. on 2004-08-02 01:45:06