Pipe A fills a tank in 2 hours; pipe B empties it in 6 hours. If both are open, how long to fill the tank?

  1. 3 hours
  2. 4 hours
  3. 1.5 hours
  4. 5 hours
Answer

A) 3 hours

Net filling per hour = tfrac12 - tfrac16 = tfrac412, so time = 3 hours.

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