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Pipes A and B can fill a tank in 5 and 6 hours respectively. Pipe C can empty it in 12 hours. If all the three pipes are opened together, then the tank will be filled in:

a) $$1\frac{{13}}{{17}}$$ hours
b) $$2\frac{8}{{11}}$$ hours
c) $$3\frac{9}{{17}}$$ hours
d) $$4\frac{1}{2}$$ hours

Answer: c
Explanation:
$$\eqalign{ & {\text{Net}}\,{\text{part}}\,{\text{filled}}\,{\text{in}}\,{\text{1}}\,{\text{hour}} \cr & = {\frac{1}{5} + \frac{1}{6} – \frac{1}{{12}}} = \frac{{17}}{{60}} \cr & {\text{The}}\,{\text{tank}}\,{\text{will}}\,{\text{be}}\,{\text{full}}\,{\text{in}}\,\frac{{60}}{{17}}\,{\text{hours}} \cr & i.e.,\,3\frac{9}{{17}}\,{\text{hours}} \cr} $$
Difference between their ages
$$\eqalign{ & = \left( {8x – 7x} \right) \cr & = {\text{ }}x\,{\text{years}} \cr & {\text{ = 2}}\,{\text{years}} \cr} $$

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