Multiple choice

Two pipes can fill a cistern in 3 hours and 4 hours respectively and a waste pipe can empty it in 2 hours. If all the three pipes are kept open, then the cistern will be filled in

  1. 5 hours

  2. 8 hours

  3. 10 hours

  4. 12 hours

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

Fill rates: 1/3 and 1/4 per hour. Empty rate: 1/2 per hour. Net rate = 1/3 + 1/4 - 1/2 = (4+3-6)/12 = 1/12. So cistern fills in 12 hours. Option C (10 hours) is a distractor - possibly from averaging the fill rates without proper calculation.