math
posted by Jon .
9 squares and the 5 is in the middle, fill in the other squares using 19 to make all columns, rows, and diagonals equal.

ah yes, the classic oddorder magic square.
Place 1 in the middle of the top row. Move upward diagonally, counting as you go. Wrap across at boundaries. Move down when you can't move due to a full square or a corner boundary:
8 1 6
3 5 7
4 9 2