A water tank can be filled in 12 hours by an inlet pipe and emptied in 30 hours by an outlet pipe. How long will it take to fill the tank if both pipes are left open?
A)60 hours
B)21 hours
C)30 hours
D)42 hours
E)20 hours

Question

Quizplus · Accepted Answer

The inlet pipe fills 1/12 of the tank per hour, and the outlet pipe empties 1/30 of the tank per hour. The net fill rate is (1/12 - 1/30) = 1/20 of the tank per hour. Therefore, it will take 20 hours to fill the tank.

A Water Tank Can Be Filled in 12 Hours by an Inlet