Two pipes 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 :

Two pipes 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]10 hours
[B]12 hours
[C]5 hours
[D]8 hours

12 hours
Part of the cistern filled in 1 hour = $latex \frac{1}{3}+\frac{1}{4}-\frac{1}{2}&s=1$
(Cistern filled by 1st pipe + Cistern filled by 2nd pipe – Cistern emptied by 3rd pipe )
$latex =>\frac{4+3-6}{12}=\frac{1}{12}&s=1$
Hence, the cistern will be filled in 12 hours.
So option [B] is the right answer.


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