Water in a rectangular reservoir having base $$80m$$ by $$60m$$ is $$6.5m$$ deep. In what time can the water be emptied by a pipe of which the cross-section is a square of side $$20cm$$, if the water runs through the pipe at the rate of $$15km/hr$$?
Let the reservoir be emptied out in $$t$$ hours. Then
$$\cfrac{20}{100}\times \cfrac{20}{100}\times 15000\times t=80\times 60\times 6.5$$
$$\Rightarrow$$ $$t=52$$ hours.