A pipe can fill a tank in 20 minutes, but the tank has a leak that can drain it completely in 28 minutes. If both the filling pipe and the leak are active simultaneously, how long will it take to fill the tank?

Explanation

The filling rate is 1/20 tank per minute, and the leak empties at 1/28 tank per minute. The net filling rate is (1/20) - (1/28) = 1/70 tank per minute. Therefore, the tank will be filled in 70 minutes.