[4.8 min] A water tank is two-fifth full. Pipe A can fill a tank in 12 minutes and pipe B can empty it in 6 mi

For Competitive Exams

A water tank is two-fifth full. Pipe A can fill a tank in 12 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?

2.8 min

4.2 min

4.8 min

5.6 min

Explanation:

Since pipe B is faster than pipe A, the tank will be emptied.

Part filled by pipe A in 1 minute $=\dfrac{1}{12}$

Part emptied by pipe B in 1 minute $=\dfrac{1}{6}$

Net part emptied by pipe A and B in 1 minute

$=\dfrac{1}{6}-\dfrac{1}{12}=\dfrac{1}{12}$

Time taken to empty $\dfrac{2}{5}$ of the tank

$=\dfrac{\left(\dfrac{2}{5}\right)}{\left(\dfrac{1}{12}\right)}$=$\dfrac{2}{5}$×12=4.8 min

Additional Questions

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