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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:

60 gallons
100 gallons
120 gallons
180 gallons
Explanation:

Work done by the waste pipe in 1 minute =$ \dfrac{1}{15} $-$ \left(\dfrac{1}{20} +\dfrac{1}{24} \right) $

    =$ \left(\dfrac{1}{15} -\dfrac{11}{120} \right) $

    = - $ \dfrac{1}{40} $.    [-ve sign means emptying]

$\therefore$ Volume of$ \dfrac{1}{40} $part = 3 gallons.

Volume of whole = $\left(3 \times 40\right)$ gallons = 120 gallons.

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