A water tank can be filled in 28 hours by an inlet pipe and emptied in 36 hours by an outlet pipe. How long will it take to fill the tank if both pipes are left open?
A) 64 hours
B) 32 hours
C) 126 hours
D) 36 hours
E) 72 hours

Question

Quizplus · Accepted Answer

The inlet pipe fills the tank at a rate of 1/28 of the tank per hour, and the outlet pipe empties it at a rate of 1/36 of the tank per hour. When both pipes are open, the net rate of filling the tank is (1/28 - 1/36) of the tank per hour. Finding a common denominator (252), this becomes (9/252 - 7/252) = 2/252 = 1/126 of the tank per hour. Therefore, it will take 126 hours to fill the tank.

A Water Tank Can Be Filled in 28 Hours by an Inlet