Inspired by martian, here is a new math puzzle (not from facebook). I used his same numbers and added a new one at the bottom to make sure we had all of the numbers. You can give the answer, but if you figure it out, give it a day or two before posting the solution.

8709 = 6
7112 = 2
1172 = 2
6666 = 6
1711 = 1
3213 = 9
7662 = 3
9312 = 6
0000 = 0
2222 = 8
3333 = 3
5555 = 2
8193 = 3
8098 = 7
7777 = 1
9999 = 9
7756 = 7
6855 = 6
9881 = 8
5531 = 5
4387 = 4
2581 = ?