Question

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:

• A. 80 min
• B. 70 min
• C. 60 min
• D. 48 min

Answer 1

Answer:

B. 70 min

Solution: Given: 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.