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 re: full solution by Mike)

Mike (or Charlie), YOU are seeing a 5 and a 7 !!!!

My solution analyses how is YOUR reasoning if you had a 3 (immediately eliminated) or a 5 or a 7, and eliminates all of them.

And rests only the solution (5, 7, 11), the 11 being in YOUR forehead.

I think both of you didn't get my solution.

Posted by pcbouhid on 2005-03-11 21:44:45