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1. A pipe can fill a tank in x hours and another pipe can empty it in y (y > x) hours. If both pipes are open, in how many hours will the tank is filled?

A. (x - y) hours

B. (y - x) hours

C. $\frac{x y}{x - y}$ hours

D. $\frac{x y}{y - x}$ hours

Solution:

Net part filled in 1 hour

\begin{aligned} = (\frac{1}{x} - \frac{1}{y}) \\ = (\frac{y - x}{x y}) \end{aligned}

∴ The tank will be filled in

= (\frac{x y}{y - x}) hours

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