Pipe A is twice as fast as pipe B. Together they fill a tank in 12 hours. How long does A take alone?

Explanation

1. 01

   A does 2 parts of the work for every 1 of B, so A does $\frac{2}{3}$ of the combined rate: $\frac{2}{3} \times \frac{1}{12} = \frac{1}{18}$.

2. 02

   A alone takes 18 hours (and B 36).