Pipe A fills a tank in 6 hours and pipe B fills it in 3 hours. How long do they take together?

Explanation

1. 01

   In one hour A fills $\frac{1}{6}$ and B fills $\frac{1}{3}$ of the tank: together $\frac{1}{6} + \frac{2}{6} = \frac{1}{2}$.

2. 02

   Half a tank an hour fills it in 2 hours.