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(4): Trying to solve this... by Tristan)

Tristan, you said :

The reasoning of my guessing (I'm the one who are seeing a 5 and a 7), is based in the fact that each of the other two (that are reasoning in a similar way), could not announce his number, because they were in doubt, and so I had enough time to figure out my own number.

My proposal is : I'm brazilian, as you already know. Perhaps in my explanation, my english is not too good for supporting my solution (could, would, have, had, ...). So, if somebody who are "commenting" this problem knows "portuguese", I could send my solution in my idiom, and he translates into english in a way that makes it clear, with no doubt.     

Posted by pcbouhid on 2005-03-14 16:53:47