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.