If 3% of (a + b) = 7% of (ab) and 5% of (a − b) = 4% of (ab), then what percentage of b is a?

For a meaningful percentage b is nonzero, and the first equation then excludes a=0. Equating ab gives 3(a+b)/7=5(a−b)/4. Hence 12(a+b)=35(a−b), so 23a=47b and 100a/b=4700/23%.