Pipe A fills a tank in 10 hours and pipe B fills it in 15 hours. How long do they take together?

Explanation

1. 01

   Rates add: $\frac{1}{10} + \frac{1}{15} = \frac{1}{6}$ of the tank per hour.

2. 02

   Together: $6$ hours. (Shortcut: $\frac{ab}{a + b} = \frac{150}{25}$.)