Answer:
start by defining some extremes:
the absolute maximum for a/b would be at the maximum a and minimum b. This is 4/4 = 1. The minimum for a/b is 3/5
Are these both allowed values? NO! Each case is a limit which cannot be reached, because the a and b values must stop short of the limits. The answer appears to be: 3/5 < a/b < 1 (0.6 < a/b < 1)