Two pipes together fill a tank in 12 minutes. One of them alone fills it in 20 minutes. How long does the other take alone?

Explanation

1. 01

   The other pipe's rate $= \frac{1}{12} - \frac{1}{20} = \frac{5 - 3}{60} = \frac{1}{30}$ of the tank a minute.

2. 02

   So it takes 30 minutes alone.