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 solution by gary wallravin)

1,5,7 is not a valid combination for sides of a triangle...bcoz to form a triangle, the sum of two sides must be greater than the third side...in this case, 1+5 is not greater than 7 so you cannot form a triangle using this combination...my solution would be 5,7,11...to me it makes sense bcoz 11 is a prime number and the perimeter which is 5+7+11=23 is a prime number...and with this combination you can form a triangle bcoz 5+7>11, 7+11>5 and 5+11>7...but i may be wrong of course...

Posted by em on 2005-03-21 02:20:42