Multiple choice

Two pipes A and B can fill a cistern in 12 hours and 15 hours respectively. If the pipe A is opened at 6 p.m , then after how much time (in hours) should B be opened so that the cistern is full at 2:00 a.m?

  1. 2.5

  2. 4

  3. 1

  4. 2

  5. 3
Reveal answer Fill a bubble to check yourself
E Correct answer
Explanation

From 6pm to 2am is 8 hours. A fills 1/12 per hour, B fills 1/15 per hour. Let B work for t hours. Then (8/12) + (t/15) = 1 (full tank). So 2/3 + t/15 = 1, meaning t/15 = 1/3, so t = 5 hours. But B starts after A, so B opens at 2am - 5hrs = 9pm, which is 3 hours after 6pm. So B should be opened after 3 hours.

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