Multiple choice

Two pipes A and B can fill a tank in 28 minutes and 42 minutes respectively. Another pipe can empty that tank in 42 minutes. If all three are opened simultaneously, in what time tank will be filled?

  1. 30 min.

  2. 35 min.

  3. 28 min.

  4. 42 min.

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

Pipe A fills at 1/28 tank per minute, pipe B fills at 1/42 tank per minute, and the emptying pipe removes at 1/42 tank per minute. Combined rate = 1/28 + 1/42 - 1/42 = 1/28 tank per minute. Therefore, the tank fills in 28 minutes. The emptying pipe cancels out pipe B's contribution exactly.

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