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-

Answer: Option D

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 )
$$\begin{array}{rl}& \frac{4+3-6}{12}\\ & =\frac{1}{12}\end{array}$$
Hence, the cistern will be filled in 12 hours.