1.  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

Answer: Option C

Explanation:

Work done by the waste pipe in 1 minute 


 =1-1+1
152024


    = 1 - 11
15 120

    = -  1 .    [-ve sign means emptying]
40

Volume of 1 part = 3 gallons.
40

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