Multiple choice

A cistern is filled with water by an inlet pipe in 15 minutes. While it becomes empty in 20 minutes by an outlet pipe. If both pipes are opened simultaneously, then the cistern will be filled in

  1. 1/2 hr.

  2. 1/4 hr

  3. 1 hr

  4. 2 hrs
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

Inlet fills in 15 min (rate = 1/15 per min), outlet empties in 20 min (rate = -1/20 per min). Combined rate = 1/15 - 1/20 = 4/60 - 3/60 = 1/60 per min. Time to fill = 60 min = 1 hour. Option C is correct.

Keep practicing — related questions