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 full solution by pcbouhid)

I guess we will have to discuss this. This seems to be the right solution. People who are saying that "different" prime numbers should ne mentioned in the problem should take a note that: in that case there is no need for the argument that whether they can or cannot guess the number on their forehead. Because we are left with just one choise which is 11.

I feel Pcbouhid had put some interesting thoughts... i was trying to think on the same lines but couldnt come up with the solution. Great Job pcbouhid

Posted by Sachin on 2005-03-11 23:18:29