A tank normally fills in 10 hours, but because of a leak at the bottom it takes 12 hours. How long would the leak take to empty the full tank?

Explanation

1. 01

   Leak rate $= \frac{1}{10} - \frac{1}{12} = \frac{1}{60}$ of the tank an hour.

2. 02

   So the leak alone empties it in 60 hours.