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A water tank is two-fifth full.Pipe A can fill a tank in 10 minutes and pipe B can empty it in 6 minutes.If both the pipes are open,how long will it take to empty or fill the tank completely?

6 min.to empty
6 min.to fill
9 min.to empty
9 min.to fill

Correct Answer: 6 min.to empty

Explanation:

Clearly,pipe B is faster than pipe A and so,the tank will be emptied.

part to be emptied = 2/5

part emptied by (A+B) in 1 minute= $\frac{1}{6} - \frac{1}{10} = \frac{1}{15}$

$\frac{1}{15} : \frac{2}{5} : : 1 : x$

$\frac{2}{5} * 15 = 6 m i n s$

so, the tank will be emptied in 6 min

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