Starting with 0 in the upper left corner, enter the numbers in order, left to right, going back to the left of the next row each time you have entered the last number in a row.
Swap the 15 and the 0. Swap the 5 and the 10. Swap the 6 and the 9. Swap the 3 and the 12. Done.
0 1 2 3
4 5 6 7
8 9 10 11
12 13 14 15
15 1 2 12
4 10 9 7
8 6 5 11
3 13 14 0
Magic constant is 30. Note that the 4 corners add up to 30 as well as each group of 4 in the four corners and the group of 4 in the center.
John

My calculator said it, I believe it, that settles it