Multiple choice

Pipe A fills a swimming pool in 4 hours. Pipe B empties the pool in 6 hours. If pipe A was opened at 8:00 am and pipe B at 9:00 am, at what time will the pool be full?

  1. 15:00

  2. 17:00

  3. 18:00

  4. 19:00

  5. 20:00
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

Pipe A fills 1/4 per hour. Pipe B empties 1/6 per hour. From 8:00 to 9:00, A fills 1/4. At 9:00, remaining is 3/4. Net rate = 1/4 - 1/6 = 1/12 per hour. Time to fill remaining = (3/4) / (1/12) = 9 hours. 9:00 + 9 hours = 18:00.

AI explanation

Using the least common multiple method, let the total capacity of the pool be 12 units, meaning pipe A fills at 3 units per hour and pipe B empties at 2 units per hour. From 8:00 am to 9:00 am, pipe A alone fills 3 units. From 9:00 am onwards, both pipes operate simultaneously, giving a net filling rate of 3 - 2 = 1 unit per hour. To fill the remaining 9 units at this net rate requires 9 hours, so adding this to 9:00 am gives a final filling time of 18:00.

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