Pipes A and B fill a tank in 20 and 30 minutes. Both are opened together, and B is closed after a while, so that the tank is full in exactly 15 minutes. After how many minutes is B closed?

Explanation

1. 01

   A runs all 15 minutes and fills $\frac{15}{20} = \frac{3}{4}$ of the tank.

2. 02

   B must fill the other $\frac{1}{4}$, at $\frac{1}{30}$ a minute: $\frac{1}{4} \times 30 = 7.5$ minutes.