I'll admit it, this one stumped me. ...I googled it. 

Did you have "math" and "calculations" in your head too?

The answer is:

6782 = 3

For each line, how many circles are there?  ...there's one in 6, and two in 8.

Look again at a few of them, and see:

7756 = 1
7777 = 0
6855 = 3
9999 = 4
9881 = 5
5531 = 0

Stupid kids!! 

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