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.