Correct Answer: Option C
Work done by the waste pipe in 1 minute = 1/15 (1/20 + 1/24)
=(1/15 - 11/120)
= - ¼ [-ve sign means emptying]
So, Volume of 1/40 part = 3 gallons.
Volume of whole = (3 x 40) gallons = 120 gallons.
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