Mary sent us her solution:

If you add up the numbers in each row, in each column, and in each long diagonal, you always get the same answer. It's a magic square. Durer put a magic square in one of his pictures
Also, if you pick a number and look for 17 minus it, then look at other pairs that add up to 17, it's symmetrical.

The 5x5 magic square must be filled in like this (I did it one step at a time, seeing what was possible, and this was the only way that worked):

23	6	19	2	15
10	18	1	14	22
17	5	13	21	9
4	12	25	8	16
11	24	7	20	3