Each inlet pipe can fill an empty cistern in 84 hours while each drain pipe…
2023
Each inlet pipe can fill an empty cistern in 84 hours while each drain pipe can empty the same cistern from a filled condition in 105 hours. When the cistern is empty, 9 inlet pipes and 10 outlet pipes are simultaneously opened. After how many hours will the cistern be completely filled?
- A.
84
- B.
88
- C.
80
- D.
90
Attempted by 5 students.
Show answer & explanation
Correct answer: A
Nine inlet pipes fill 9/84 = 3/28 of the tank per hour, and ten drain pipes empty 10/105 = 2/21 per hour. Net rate = 3/28 - 2/21 = 1/84, so the cistern fills in 84 hours.