Pipe A fills a tank in 12 hours and pipe B in 18 hours. Both are open for 4 hours. What part of the tank is filled?

Explanation

1. 01

   Together per hour: $\frac{1}{12} + \frac{1}{18} = \frac{3 + 2}{36} = \frac{5}{36}$.

2. 02

   In 4 hours: $\frac{20}{36} = \frac{5}{9}$.