PUZZLE

I copied the following puzzle from a website called gawkerassets.  Take a shot at solving it.

This problem can be solved by pre-school children in five to ten minutes, by programmers in an hour and by people with higher education... well, check it yourself!

8809 = 6     7111 = 0     2172 = 0     6666 = 4
1111 = 0     3213 = 0     7662 = 2     9313 = 1
0000 = 4     2222 = 0     3333 = 0     5555 = 0
8193 = 3     8096 = 5     1012 = 1     7777 = 0
9999 = 4     7756 = 1     6855 = 3     9881 = 5
5531 = 0     2851 = ???

It took me about 15 or 20 minutes to come up with the answer.  If you can solve it, send it in a comment and pat yourself on the back.  No fair asking a pre-schooler for help.   I'll publish the explanation next Wednesday.

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