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: centering the point by pcbouhid)

Pcbouid:

As I said before I think that your solution is fine. Let me say that I think it is complet in order at the problem post.

I ment only in my mail that we have assumed that 11 is a solution to the problem, proceeding by way of exclusion: the other numbers couldn't be, it is stated that there is a solution, so the only number that remains is the solution.

But we haven't verify that 11 is a solution, appliying to 11 the same approach used to the other primes. Sure: we are not obliged to, but can be done...

It was only this. It's not important. Armando.

Posted by armando on 2005-03-14 21:46:09