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Pipes A and B can fill a tank in 8 and 24 hours respectively. Pipe C can empty it in 12 hours. If all the three pipes are opened together, then the tank will be filled in:

18 hr
6 hr
24 hr
12 hr
Explanation:

Part filled by pipe A in 1 hr $=\dfrac{1}{8}$

Part filled by pipe B in 1 hr $=\dfrac{1}{24}$

Part emptied by pipe C in 1 hr $=\dfrac{1}{12}$

Net part filled by pipe A, B, C together in $1$ hr

$=\dfrac{1}{8}+\dfrac{1}{24}-\dfrac{1}{12}=\dfrac{2}{24}=\dfrac{1}{12}$

i.e, the tank will be filled in $12$ hr.

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