You and two other people have numbers written on your foreheads. You are all told that the three numbers are primes and that they form the sides of a triangle with a prime perimeter. You see 5 and 7 on the other two heads and both of the other people agree that they cannot deduce the number on their own foreheads.

What is the number written on your forehead?

(In reply to Charlie's Skepticism by Eric)

The usual problem of this type involves discrete time frames, such as "the morning of day 1" at which all participants announce if they can figure out the answer. Here there are no discrete time frames that pass by.

Also it is not stated that the person making the decision ("you" in the language of the puzzle) make it clear to the other two that "you" in fact cannot deduce the answer; it is only stated that the other two agree that they cannot deduce the answer.

Posted by Charlie on 2005-03-12 14:09:04