Multiple choice

A cistern has two pipes connected with it. One can fill it in 8 hours and other can empty it in 5 hours. In how many hours will the cistern be emptied if both the pipes are opened together when 3/4 of the cistern is already full of water?

  1. 5

  2. 10

  3. 7

  4. 12

Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

The fill pipe fills at 1/8 cistern per hour, the empty pipe empties at 1/5 per hour. Combined rate = 1/8 - 1/5 = -3/40 per hour (emptying). To empty 3/4 of the cistern: Time = (3/4) / (3/40) = 10 hours. The negative sign indicates emptying.