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Two pipes can fill a tank in 20 and 24 minutes respectively and a waste pipe can empty 3 gallons per minute. All the three pipes working together can fill the tank in 15 minutes. The capacity of the tank is:

a) 60 gallons
b) 100 gallons
c) 120 gallons
d) 180 gallons

Answer: c
Explanation: Work done by the waste pipe in 1 minute
$$\eqalign{ & = \frac{1}{{15}} – \left( {\frac{1}{{20}} + \frac{1}{{24}}} \right) \cr & = {\frac{1}{{15}} – \frac{{11}}{{120}}} \cr & = – \frac{1}{{40}}\,\,\,\,\,\left[ { – ve\,{\text{sign}}\,{\text{means}}\,{\text{emptying}}} \right] \cr & {\text{Volume}}\,{\text{of}}\,\frac{1}{{40}}{\text{part}} = 3\,{\text{gallons}} \cr & {\text{Volume}}\,{\text{of}}\,{\text{whole}} \cr & = \left( {3 \times 40} \right){\text{gallons}} \cr & = 120\,{\text{gallons}} \cr} $$

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