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?

As the lengths of any two sides of a triangle must exceed the length of the third side, unless there is some unstated caveat then the smallest prime that will accomplish the task is 11.  However, why the other two couldn't find the answer poses an element of uncertainty

Posted by Brunner, J on 2005-03-22 14:51:23