Fill a 3x3 square with the numbers from 1 to 9, so the four sides and the two diagonals share the same sum, which is to be as large as possible.
6 5 7
4 3 2 = 18
8 1 9
This is the only square with sides and diagonals equal to each other. There are, of course, reflections and rotations of this square.

Posted by Erik O.
on 20040622 09:24:52 