N is a 3x3 square grid which is constituted by using each of the digits from 1 to 9 exactly once.

Determine the probability that the first digit minus the second digit plus the third digit in each row (reading left to right), each column (reading top to bottom), and each main diagonal (reading top to bottom) of N is the same.

Brian:  Thanks for the congratulations, but I can't take full credit. Sometimes, the analytical solution is easier after the computer guys come up with the answer.  That was certainly the case here, where I could see that all solutions had the same center square and the same constant.  It gave me a hint of what to solve for.  

Posted by Steve Herman on 2009-12-06 22:15:38