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

Answer: Option D

Solution(By Examveda Team)

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