Besides finding missing numbers in a problem, you can also find missing symbols. For instance consider this problem.
5 __ 5 = 10
Here are some more examples.
PROBLEM MISSING SYMBOL
-------------------+------------------------
5 __ 2 = 7 | +
5 __ 2 = 3 | -
10 __ 11 = 21 | +
21 __ 11 = 10 | -
45 __ 15 = 30 | -
22 __ 23 = 45 | +
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