Two pipes can fill a cistern separately in 10 hours and 15 hours. They can together fill the cistern in

Two pipes can fill a cistern separately in 10 hours and 15 hours. They can together fill the cistern in
[A]9 hours
[B]7 hours
[C]6 hours
[D]8 hours

6 hours
Part of the cistern filled by both pipes in 1 hour = $\frac{1}{10}+\frac{1}{15} = \frac{3+2}{30} = \frac{1}{6}$
∴ The cistern will be filled in 6 hours, hence option [C] is the right answer.