One pump fills a tank in 20 hours and another in 15 hours. when a third pump is added the tank fills in 6 hours. find out how long it takes the 3rd pump to fill the tank alone.

Every hour, the first pump fills 1/20 of the tank, and the second fills 1/15 of the tank.  If, when combined with the third pump, all three pumps fill 1/6 of the tank in an hour, the third pump fills 1/6-1/15-1/20= of the tank every hour, so it takes a total of 20 hours to fill the tank alone.

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