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?
The problem posted is fine. Perhaps it could be added about "clever guys" and "after some instants passed". As so solution is eleven. I have reasoning like pcbohuid and arrived at the same solution before watching comments. But he esplains it smart. The only point is that pchbouid assume 11 like solution (problem posted says there is a solution that the thinker can guess), but he doen't verify really (verifying it is tought).

Posted by armando
on 20050313 09:56:43 