Pipe A fills a tank in 8 hours and pipe B in 12 hours. A is opened alone, and after 2 hours B is opened as well. How long after A was opened is the tank full?

Explanation

1. 01

   In 2 hours A fills $\frac{2}{8} = \frac{1}{4}$. The rest, $\frac{3}{4}$, fills at $\frac{1}{8} + \frac{1}{12} = \frac{5}{24}$ an hour.

2. 02

   That takes $\frac{3}{4} \div \frac{5}{24} = \frac{18}{5} = 3.6$ hours, so $2 + 3.6 = 5.6$ hours in all.