[pdurrant]

I was asked to look at this puzzle after it had stumped some friends. Because it had puzzled them for several days, I decided to look at all sorts of complicated ways to solve it. I should have looked for simple methods first... and that's the only clue I'm giving at the moment.

So — what single digit number should be where the X is?

1 2 3 4
4 0 8 5
6 3 9 7
2 4 4 6
3 6 4 X
1 5 5 3
5 6 7 8