Multiple choice

Pipe A can fill a storage cistern in 12 hours, and Pipe B can fill the same cistern in 15 hours. An outlet Pipe C can empty the completely filled cis-tern in 20 hours. If all three pipes are opened simultaneously in an empty cistern, how much time will it take to fill it completely?

  1. 8 hours

  2. 10 hours

  3. 12 hours

  4. 15 hours

  5. 9 hours
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

Pipe A fills 1/12 per hour, B fills 1/15, C empties 1/20. Net rate = 1/12 + 1/15 - 1/20 = (5+4-3)/60 = 6/60 = 1/10. Time = 10 hours.

AI explanation

Using the one unit work method, the individual rates of pipes A, B, and C are 1/12, 1/15, and -1/20 of the cistern per hour, respectively. The combined rate of all three pipes is 1/12 + 1/15 - 1/20, which equals 1/10 of the cistern per hour. The total time to fill the cistern is the reciprocal of the combined hourly rate. The result is 10 hours.

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