$A$ man swimming in a stream which flows at $1.5 \, km/hr$ finds that in a given time he can swim twice as far with the stream as he can against it. At what rate does he swim in still water? (in $km/hr$)

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 $48 \ s$.

$A$ person takes total $5$ $hours$ to go to a destination downstream and come back upstream by boat. If the speed of the boat in still water is $10 \text{ km/h}$ and the speed of the stream is $4 \text{ km/h}$,what is the distance of the destination from the starting point (in $\text{km}$)?

$A$ boat goes $24 \, km$ upstream and $28 \, km$ downstream in $6 \, hours$. It goes $30 \, km$ upstream and $21 \, km$ downstream in $6 \, hours$ and $30 \, minutes$. The speed of the boat in still water is $...... \, km/hr$.

$A$ man moves downstream at a rate of $14\, km/h$ and upstream at a rate of $6\, km/h$. The speed of the boat in still water is........$km/h$.

$A$ man rows to a place $48 \, km$ away and comes back in $14 \, hours$. He finds that he can row $4 \, km$ with the stream in the same time as $3 \, km$ against the stream. The rate of the stream is: