$A$ man can row at a speed of $15\, km/hr$ in still water to a certain upstream point and back to the starting point in a river which flows at $3\, km/hr$. Find his average speed for the total journey (in $km/hr$).

The speed of a boat in still water is $7\, km/h$. If its speed in downstream is $10\, km/h$,then the speed of the stream is .......... $km/h$.

$A$ man can row $6\, km/h$ in still water. If the river is running at $2\, km/h$,it takes $3\, hours$ more to row upstream than to row downstream for the same distance. How far is the place? (in $km$)

$A$ motor boat,whose speed is $15 \text{ km/hr}$ in still water,goes $30 \text{ km}$ downstream and comes back in a total of $4 \text{ hours } 30 \text{ minutes}$. The speed of the stream (in $\text{km/hr}$) is:

$A$ boat takes half the time to travel a certain distance downstream compared to upstream. The ratio of the speed of the boat in still water to the speed of the current is:

From the given statements,decide which of the statement$(s)$ is/are necessary to answer the question.
What is the speed of a train?
$I.$ The length of the train is $240 \ m$.
$II.$ The train crosses a pole in $24 \ s$.
$III.$ The train crosses a platform in $48 \ s$.