Multiple choice

Three pipes A, B, C can fill an empty cistern in 2, 3 and 6 hours, respectively. They are opened together. After what time should B be closed, so that the cistern gets filled in exactly 1 hr 15 min?

  1. 30 min

  2. 20 min

  3. 45 min

  4. 15 min
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

A, B, C fill 1/2, 1/3, 1/6 per hour. Combined rate = 1/2 + 1/3 + 1/6 = 1. Total time = 1.25 hours. If B is closed after t hours, (1/2 + 1/6)*1.25 + (1/3)*t = 1. (2/3)*1.25 + t/3 = 1. 0.833 + t/3 = 1. t/3 = 0.166. t = 0.5 hours = 30 min.

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