A pipe can fill a tank in x hours and another can empty it in y hours. In how many hours they can together fill it (where y > x) :
[A]x-y\ hours
[B]y-x\ hours
[C]\frac{xy}{y-x}\ hours
[D]\frac{xy}{x-y}\ hours

\frac{xy}{y-x}\ hours
When both pipes are opened simultaneously, part of the tank filled in 1 hour
= \frac{1}{x}-\frac{1}{y}=\frac{y-x}{xy}
∴ Required time = \frac{xy}{y-x}\ hours
Hence option [C] is correct answer.