Multiple choice

Pipe A can fill a tank in 15 hours and Pipe B can fill it in 20 hours. Both pipes are opened together,but after 4 hours, Pipe A is closed. Find the total time (in hours) taken to fill the tank.

  1. 14 2/3

  2. 14 1/3

  3. 16 2/3

  4. 12 2/3

  5. 15 1/3
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

Pipe A rate = 1/15, Pipe B rate = 1/20. Combined rate = 7/60. In 4 hours, they fill 4 * (7/60) = 28/60 = 7/15. Remaining = 8/15. Pipe B fills this in (8/15) / (1/20) = 160/15 = 32/3 = 10 2/3 hours. Total time = 4 + 10 2/3 = 14 2/3 hours.

AI explanation

Using the one unit work method, the combined rate of pipes A and B is 1/15 + 1/20, which equals 7/60 of the tank per hour. Working together for 4 hours, they fill 4 times 7/60, or 28/60, leaving 32/60 of the tank empty. Pipe B works alone at a rate of 1/20 per hour, so the time to fill the remaining 32/60 is (32/60) divided by (1/20), which equals 32/3 hours. Adding this to the initial 4 hours gives a total time of 44/3 hours. The result is 14 2/3 hours.

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