Question

A pipe fills a tank in 5 hours and a leak empties it in 20 hours. How long will it take to fill the tank when both are open?

Solution

1. The inlet fills 1/5 of the tank per hour.
2. The leak empties 1/20 of the tank per hour.
3. Net rate = 1/5 - 1/20 = 3/20 tank per hour.
4. Time to fill the tank = 1 divided by 3/20 = 20/3 hours, or 6 hours 40 minutes.