Pipe A fills a tank in 3 hours; pipe B empties it in 6 hours. If both are open, how long to fill the tank?

  1. 8 hours
  2. 3 hours
  3. 6 hours
  4. 2 hours
Answer

C) 6 hours

Net filling per hour = tfrac13 - tfrac16 = tfrac318, so time = 6 hours.

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