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1. Two pipe can fill a cistern in 3 hours and 4 hours respectively and a waste pipe can empty it in 2 hours. If all the three pipes are kept open, then the cistern will be filled in-

A. 5 hours

B. 8 hours

C. 10 hours

D. 12 hours

Solution:

Part of the cistern filled in 1 hour

= \frac{1}{3} + \frac{1}{4} - \frac{1}{2}

(Cistern filled by 1^st pipe + Cistern filled by 2^nd pipe – Cistern emptied by 3^rd pipe )

\begin{aligned} \frac{4 + 3 - 6}{12} \\ = \frac{1}{12} \end{aligned}

Hence, the cistern will be filled in 12 hours.

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