Pipe A fills a tank in 10 hours and pipe B in 15 hours. A is opened alone, and after 2 hours B is opened as well. How long after B is opened is the tank full?

Explanation

1. 01

   In 2 hours A fills $\frac{2}{10} = \frac{1}{5}$, leaving $\frac{4}{5}$.

2. 02

   Together they fill $\frac{1}{10} + \frac{1}{15} = \frac{1}{6}$ an hour, so $\frac{4}{5} \times 6 = 4.8$ hours.