Pipe A fills a tank in 4 hours and pipe B fills it in 12 hours. How long do they take together?

Explanation

1. 01

   Rates add: $\frac{1}{4} + \frac{1}{12} = \frac{1}{3}$ of the tank per hour.

2. 02

   Together: $3$ hours. (Shortcut: $\frac{ab}{a + b} = \frac{48}{16}$.)