A pipe can fill a cistern in 20 minutes whereas the cistern when full can be emptied by a leak in 28 minutes. When both are opened,the time taken to fill the cistern is :
Show answer & explanation
Correct answer: B
Explanation
The correct answer is option 2 i.e. 70 min
Let total capacity of tank be LCM of 20 and 28 i.e 140 units
Efficiency = Total work/time
A pipe can fill a cistern in 20 minutes
Efficiency of pipe = 140/20 = 7units
An outlet pipe can empty the tank in 28hrs
Efficiency of outlet pipe = 140/28 = 5units
When both pipes are opened the net efficiency = 7 - 5 = 2 units
Tank will be filled in 140/2 = 70 hrs