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Pipe A can fill a tank in 18 minutes and pipe B can empty it in 27 minutes. If both are opened on a half-full tank, how long until the tank is full?

Competition practicePipes & Cisterns

A27 min
B54 min
C9 min
D13.5 min

Step-by-Step Solution

Net

= \frac{1}{18} - \frac{1}{27} = \frac{3 - 2}{54} = \frac{1}{54}

per minute; half takes 27 min. (54 min fills from empty; 9 min ignores B.)

Answer: A) 27 min

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