Pipes A and B can fill a tank in 20 minutes and 30 minutes respectively, while pipe C can drain the tank in 40 minutes. If all three pipes are opened simultaneously, how long will it take to fill the tank?

Explanation

The combined filling rate is (1/20) + (1/30) minus the emptying rate (1/40), which equals 7/120 of the tank per minute. Therefore, the total time to fill the tank is the reciprocal of this rate: 120/7 minutes, or 17 minutes and 1/7.