$A$ man can row $15\, km/h$ downstream and $9\, km/h$ upstream. The speed of the boat in still water is.....$km/h$

$A$ man's speed with the current is $15 \, km/hr$,and the speed of the current is $2.5 \, km/hr$. The man's speed against the current is.......$km/hr$

When the speed of a boat in still water is $4\, km/h$ and the rate of stream is $2\, km/h$,find the upstream speed of the boat (in $km/h$).

$A$ boat takes $90$ minutes less to travel $36$ miles downstream than to travel the same distance upstream. If the speed of the boat in still water is $10$ miles/hour,the speed of the stream is.......miles/hour.

$A$ man can row against the current $\frac{3}{4}$ of a kilometre in $15$ minutes and returns the same distance in $10$ minutes. The ratio of his speed to that of the current is

$A$ motor-boat can travel at $10 \, km/h$ in still water. It travelled $91 \, km$ downstream in a river and then returned to the same place,taking altogether $20 \, hours$. Find the rate of flow of the river (in $km/h$).