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=[tex] \frac{1}{20} [/tex] of the tank every hour, so it takes a total of 20 hours to fill the tank alone.
