A boat which has a speed of 5 km/hr in still water crosses a river of width 1, km along the shortest possible path in 15 minutes. The velocity of the river water in km/hr is

Let u be the velocity of river. Now the resultant velocity of the boat = [(5)² - u²]^1/2
The boat crosse the river (width 7 km) along shortest possible path in 15 minutes. Thus, velocity of boat
$\begin{array}{l}=\frac{1\mathrm{km}}{0\cdot 25\mathrm{h}}=4\mathrm{km}/\mathrm{h}\\ \therefore 4={\left[(5{)}^2-{\mathrm{u}}^2\right]}^{1/2} \mathrm{or} \mathrm{u}=3\mathrm{km}/\mathrm{h}\end{array}$