If pipe $P$ can fill a tank in $p$ hours and pipe $Q$ can empty the same tank in $q$ hours (where $q > p$),and both pipes are opened simultaneously,in how many hours $(r)$ will the tank be filled?
- A$\frac{1}{r} = \frac{1}{p} - \frac{1}{q}$
- B$\frac{1}{r} = \frac{1}{p} + \frac{1}{q}$
- C$r = p + q$
- D$r = p - q$
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