A school's water tank has two inlet pipes that fill it in 20 and 30 minutes, and an outlet that empties it in 15 minutes. If all three are opened on an empty tank, how long will it take to fill?

Explanation

1. 01

   Inlets: $\frac{1}{20} + \frac{1}{30} = \frac{3+2}{60} = \frac{5}{60}$ per minute.

2. 02

   Outlet: $\frac{1}{15} = \frac{4}{60}$ per minute.

3. 03

   Net: $\frac{5 - 4}{60} = \frac{1}{60}$, so 60 minutes.