Pipe A fills a tank in 20 minutes and pipe B fills it in 30 minutes. How long do they take together?

Explanation

1. 01

   Rates add: $\frac{1}{20} + \frac{1}{30} = \frac{1}{12}$ of the tank per minute.

2. 02

   Together: $12$ minutes. (Shortcut: $\frac{ab}{a + b} = \frac{600}{50}$.)