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