Answer is B
A+B+C=$\frac{1}{15}$ A=$\frac{1}{20}$ B=$\frac{1}{24}$, we have to find C,
$\frac{1}{A+B+C}=\frac{1}{A}+\frac{1}{B}-\frac{1}{C}$ [as C is emptying the tank, so we will give “-”]
$\frac{1}{15}=\frac{1}{20}+\frac{1}{24}-\frac{1}{C}$
On solving, we wil get C = 40min.
Given, C can empty 3 gallons in 1min,
so, in 40min, C can empty 3*40 = 120 gallons.