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