This is the back side of Rubik's fifteen puzzle.
The goal is to form a magic square with the numbers 0 to 15 and a sum of 15.. A magic square has rows, columns, and diagonal lines which have the same total. E.g.:
| 6 | 1 | 8 | This is a magic square, where all the rows, collumns and diagonal lines have a sum of 15. | |
| 7 | 5 | 3 | ||
| 2 | 9 | 4 |
Have fun solving this very interesting puzzle. But don't get too exited, because this puzzle isn't produced anymore, so it's hard to get one for yourself. On the other hand, there is a fine version on Maarten Net.
If you mastered this one, also try the front side of this puzzle.