A water tap fills a tub in 'p' hours and a sink at the bottom empties it in 'q' hours. If p < q and both tap and sink are opened the tank is filled in 'r' hours, then the relation between p, q, r :

Answer: Option B

Solution

Answer: Option B

Solution:

Net efficiency = q - p (∵ q > p)
Time required
$$\begin{array}{rl}& \text{r}=\frac{pq}{q-p}\\ & or\frac{1}{r}=\frac{1}{p}-\frac{1}{q}\end{array}$$