A water faucet can fill a sink in 10 minutes, whereas the drain can empty it in 8 minutes. If the sink is full, how long will it take to empty if both the faucet and the drain are open?
A)10 minutes
B)80 minutes
C)40 minutes
D)18 minutes
E)9 minutes

Question

Quizplus · Accepted Answer

Let the capacity of the sink be 40 units (which can be any unit of volume like liters, gallons, etc.). The faucet can fill 4 units per minute (40 units in 10 minutes) and the drain can empty 5 units per minute (40 units in 8 minutes). 
When both the faucet and the drain are open, the net inflow of water is 4-5=-1 units per minute, which means the sink will be emptied at the rate of 1 unit per minute. 
Therefore, it will take 40/1=40 minutes to empty the sink. 
So, the correct answer is choice C, 40 minutes.

A Water Faucet Can Fill a Sink in 10 Minutes