Numerical answer
Pipes A and B can fill a tank in 20 minutes and 30 minutes respectively, while pipe C can empty the full tank in 15 minutes. A and B are opened together at 9:00 a.m. on an empty tank and C is opened at 9:06 a.m. How many minutes after 9:00 a.m. will the tank be full?
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Answer: 36
Take the tank as 60 units. A fills 3 units/min, B fills 2 units/min, C empties 4 units/min.
From 9:00 to 9:06 only A and B work: 6 × 5 = 30 units (half the tank).
After 9:06 the net rate is 3 + 2 − 4 = 1 unit/min, so the remaining 30 units take 30 minutes.
Total time = 6 + 30 = 36 minutes (the tank fills at 9:36 a.m.).
Trap: Answering 30 (the time after C opens) instead of the time after 9:00.
From 9:00 to 9:06 only A and B work: 6 × 5 = 30 units (half the tank).
After 9:06 the net rate is 3 + 2 − 4 = 1 unit/min, so the remaining 30 units take 30 minutes.
Total time = 6 + 30 = 36 minutes (the tank fills at 9:36 a.m.).
Trap: Answering 30 (the time after C opens) instead of the time after 9:00.
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