Two pipes can fill a storage tank in 20 minutes and 24 minutes. A drain at the bottom lets out 3 litres per minute. With both pipes and the drain open, the empty tank fills in 15 minutes. What is the capacity of the tank?

Explanation

1. 01

   Pipes together: $\frac{1}{20} + \frac{1}{24} = \frac{11}{120}$ of the tank per minute.

2. 02

   With the drain the net is $\frac{1}{15} = \frac{8}{120}$, so the drain removes $\frac{3}{120} = \frac{1}{40}$ of the tank per minute.

3. 03

   $\frac{1}{40}$ of the tank = 3 litres, so the tank holds $3 \times 40 = 120$ litres.