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 full solution by pcbouhid)

Sounds like one too many levels of sophistication, since the puzzle never states that the narrator made clear to the others that he wasn't immediately able to identify his number--only that the other two agreed (presumably with each other) that they couldn't identify their numbers.

Posted by Charlie on 2005-03-11 20:35:47