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?

I think that i  have found  the answer to  the riddle:prime  perimeter!

it say that three numbers are listed  on each others heads.Two of the numbers  are  5 and 7.It also  says that  the  total perimeter  is a prime  number.  I think that  shape  is a right  triangle ,  and not   an equalateral. The numbers imply that, since they are not equal.5 plus seven equals 12. the bottom halve will end up as 5 if you do the math . 5+5+7=17, which just happens to be a prime number! So....17  is the final answer!!

Posted by jason on 2005-03-26 02:58:33