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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

Solution:

Work done by the waste pipe in 1 minute

\begin{aligned} = \frac{1}{15} - (\frac{1}{20} + \frac{1}{24}) \\ = \frac{1}{15} - \frac{11}{120} \\ = - \frac{1}{40} [- v e sign means emptying] \\ ∴ Volume of \frac{1}{40} part = 3 gallons \\ Volume of whole \\ = (3 \times 40) gallons \\ = 120 gallons \end{aligned}

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