Work done by the waste pipe in 1 minute = 1/15 - (1/20 + 1/24) = - 1/40
Volume of 1/40 part = 4 gallons\
Volume of whole = 4 * 40 = 160 gallons.
Two pipes can fill a tank in 20 and 24 minutes respectively and a waste pipe can empty 4 gallons per minute. All the three pipes working together can fill the tank in 15 minutes. The capacity of the tank is?

[a] 60 gallons
[b] 100 gallons
[c] 120 gallons
[d] 160 gallons
[e] 130 gallons