Q. A pipe can fill a tank in 'x' hours and another pipe can empty it in 'y' hours (y > x). If both the pipes are open, in how many hours will the tank be filled?
Answer: $latex \frac{xy}{y-x} hours &s=1$
Notes: Part of the tank filled in 1 hour = $latex \frac{1}{x}&s=1$ Part of the tank emptyied in 1 hour = $latex \frac{1}{y}&s=1$ Part of the tank filled in 1 hour when both are opened = $latex \frac{1}{x}-\frac{1}{y}= \frac{y-x}{xy}&s=1$ ∴ Tank will be filled in $latex \frac{xy}{y-x}&s=1$ hours Hence option [C] is the right answer

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