Question 745076
One pipe can fill a tank in 5 hours and another pipe can fill the same tank in 4 hours. A drainpipe can empty the full content of the tank in 20 hours. With all the three pipes open, how long will it take to fill the tank?
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In each hour, the 1st pipe adds 1/5 of the tank, the 2nd adds 1/4 of the tank, and the drain adds -1/20 of the tank.
--> (1/5 + 1/4 - 1/20) tank/hour
Hours/tank = 1/(1/5 + 1/4 - 1/20)