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 see a lot of discussion around what the person (B) who saw 5,5 thinks without examining the possibility that he has a 9.  It seems everyone is still only thinking the 3,5,7,11.  If B only sees 5,5, he can not eliminate it down to the 3,5,7,11 possibility that I can by seeing 5,7.

Similarly person (C) who sees 7,7...could have a 9 or a 13.

As I am the only one considering this I may be wrong.  But can anyone point out why, If i am indeed wrong?

Posted by john on 2005-03-14 15:24:47