Multiple choice

Two pipes can fill a cistern in 20 minutes and 30 minutes respectively. Both the pipes being opened when the cistern is empty. Find the time when the first pipe must be turned off, so that the cistern may be filled in 18 minutes?

  1. 12 minutes

  2. 10 minutes

  3. 6 minutes

  4. 8 minutes

  5. None of these
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

Let first pipe work for t minutes, then only second pipe works for (18-t) minutes. Combined rate of both pipes = 1/20 + 1/30 = 1/12 per minute. Work equation: t/12 + (18-t)/30 = 1. Solving gives t/20 + 3/5 = 1, so t/20 = 2/5, t = 8 minutes. First pipe should be closed after 8 minutes.

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