Explain simply why (A) is the correct answer to: 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?
Options:
(A) 40 gallons
(B) 100 gallons
(C) 120 gallons
(D) 180 gallons
(E) 130 gallons. Explanation:

Show your work and conclude with the exact phrasing "Therefore, the final answer is [answer]." where [answer] is just the final number or expression that solves the problem.
user
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.