A pipe can fill a cistern in 20 minutes, but an outlet can empty it in 28 minutes. When both the pipes are opened, in what time is the cistern filled?

Correct option is D

A pipe can fill a cistern in 20 minutes.
Time taken by the leak to empty the tank = 28 minutes.
Concept used:
If a pipe can fill a tank in x hours then it can fill 1/x part of the tank in 1 hour.
Calculation:
Let the time taken by a pipe to fill the tank when the leakage arises be 1 minutes is 1/20 part.
And the time taken by a pipe to empty the tank be 1 minutes is 1/28 part. When both pipes are opened,
The tank fills in 1 minute = 1/20-1/28
=> (7-5)/140
=>2/140
=> 1/70 part
The time taken to fill the tank is 70 minutes.