A full tank has an outlet that empties it in 20 hours and an inlet that fills it in 30 hours. Both are opened. How long does the full tank take to empty?

Explanation

1. 01

   The outlet is faster, so the level falls: net emptying rate $= \frac{1}{20} - \frac{1}{30} = \frac{1}{60}$ of the tank an hour.

2. 02

   The full tank empties in 60 hours.