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Reverse Subtraction

Stage: 3 Short Challenge Level: Challenge Level:1

c is 3.

There are five possible pairs of values for a and b (assuming a cannot be zero).
They are (1,5), (2,6), (3,7), (4,8) and (5,9).

Note that the pair (3,7) means a = c.
Perhaps you would prefer not to include this but it is not strictly ruled out by the problem.

All give c as 3. Why?

This problem is taken from the UKMT Mathematical Challenges.
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