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A Cistern is filled by pipe A in 8 hrs and the full Cistern can be leaked out by an exhaust pipe B in 12 hrs. If both the pipes are opened in what time the Cistern is full?

12 hrs
24 hrs
16 hrs
32 hrs
Explanation:

Pipe A can fill $\dfrac{1}{8}$ of the cistern in 1 hour.

Pipe B can empty $\dfrac{1}{12}$ of the cistern in 1 hour

Both Pipe A and B together can effectively fill $\dfrac{1}{8}-\dfrac{1}{12}=\dfrac{1}{24}$ of the cistern in 1 hour

i.e, the cistern will be full in 24 hrs.

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