Q. 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 :
Answer: 12 hours
Notes: 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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