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 number must be five also. "1" is not properly called prime, and 7+5=12 therefore, the prime number must be less than 12. I deduce that it is 5 because it is the only one that equals a prime number (17). however 11 also works but who's to say it's a double digit? you're question is too vague.

Posted by rachael on 2005-03-18 02:56:18