Two pipes can fill a tank in 20 and 24 minutes respectively and a waste pipe can empty 3 gallons per minute. All the three pipes working together can fill the tank in 15 minutes. The capacity of the tank is:

Work done by the waste pipe in 1 minute = $\frac{1}{15}-\frac{1}{20}+\frac{1}{24}=\frac{1}{15}-\frac{11}{120} = -\frac{1}{40}$ [-ve sign means emptying]

Volume of  $\frac{1}{40}$part = 3 gallons

Volume of whole = (3 x 40) gallons = 120 gallons.