The speed of a boat in still water is $8\, km/h$. If its speed downstream is $15\, km/h$,then the speed of the stream is $.......\, km/h$.

$A$ man can row against the current $3/4$ of a kilometre in $15$ $min$ and returns the same distance in $10$ $min.$ The ratio of his speed to that of the current is:

$A$ man can row at the rate of $3.5 \, km/h$ in still water. If the time taken to row a certain distance upstream is $2 \frac{1}{2}$ times as much as the time taken to row the same distance downstream,find the speed of the current in $km/h$.

The ratio of the speed of a motorboat to that of the water current is $36 : 5$. The motorboat travels along with the current in $5 \, h \, 10 \, min$. Find the time taken by the motorboat to return.

The current of a stream runs at the rate of $4 \text{ km/h}$. $A$ boat goes $6 \text{ km}$ and comes back to the starting point in $2 \text{ hours}$. The speed of the boat in still water is ....... $\text{km/h}$.

If a boat goes upstream at a speed of $21\, km/h$ and comes back the same distance at $28\, km/h$,what is the average speed (in $km/h$) for the total journey?