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?

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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?

A. 60 gallons
B. 100 gallons
C. 120 gallons
D. 180 gallons
Explanation:
Work done by the waste pipe in 1 minute = 1/15 – (1/20 + 1/24) = – 1/40
Volume of 1/40 part = 3 gallons
Volume of whole = 3 * 40 = 120 gallons.