Multiple choice

Two pipes can fill a tank 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 tank 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

Two fill pipes: 1/3 and 1/4 of tank per hour. Waste pipe: 1/2 of tank per hour (emptying). Net rate when all open: 1/3 + 1/4 - 1/2 = (4+3-6)/12 = 1/12 of tank per hour. Time to fill = 1 ÷ (1/12) = 12 hours.