A pipe can fill a cistern in 20 minutes where as 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
A pipe can fill a cistern in 20 minutes & it can be emptied by a leak in 28 minutes.
Let the capacity of the cistern be LCM (28, 20) = 140 units
⇒ Efficiency of pipe = 140/20 = 7 units
⇒ Efficiency of leak = 140/28 = -5 units
⇒ 7 - 5 = 2 units
⇒ Time taken = 140/2 = 70 min