The speed of a swimmer is 4 km h^-1 in still water. If the swimmer makes his strokes normal to the flow of river of width 1 km, he reaches a point…

The speed of a swimmer is $\displaystyle 4 \mathrm{~km} \mathrm{~h}^{-1}$ in still water. If the swimmer makes his strokes normal to the flow of river of width $\displaystyle 1$ km , he reaches a point $\displaystyle 750$ m down the stream on the opposite bank.The speed of the river water is $\displaystyle \_\_\_\_$ $\displaystyle \mathrm{km} \mathrm{h}^{-1}$