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for x-y to be greatest, x needs to be greatest and y needs to be least
b/3a is greatest when bis big and a is small: 5/9
a/3b is least when a is small and b is big: 3/15 = 1/3
so b/3a - a/3b is greatest at 5/9 - 1/3 = 2/9
Now find when b/3a - a/3b is least, and then you have your range of values.