Multiple choice

Two pipes can fill a cistern in 72 and 90 minutes respectively. If both the pipes are left open how long will it take for the cistern to be half full?

  1. 40 minutes

  2. 24 minutes

  3. 48 minutes

  4. 20 minutes

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

Combined rate = 1/72 + 1/90 = 5/360 + 4/360 = 9/360 = 1/40. So full cistern takes 40 minutes together. Half full takes 40/2 = 20 minutes.