Work done by the waste pipe in 1 minute = 1/15 - (1/20 + 1/24) = - 1/40
Volume of 1/40 part = 1 gallons\
Volume of whole = 1 * 40 = 40 gallons.
Two pipes can fill a tank in 20 and 24 minutes respectively and a waste pipe can empty 1 gallons per minute. All the three pipes working together can fill the tank in 15 minutes. The capacity of the tank is?
[1] 40 gallons
[2] 100 gallons
[3] 120 gallons
[4] 180 gallons
[5] 130 gallons