$A$ person can row with the stream at $8 \text{ km/h}$ and against the stream at $4 \text{ km/h}$. The speed of the current is (in $\text{km/h}$):

$A$ boat travels upstream from $B$ to $A$ and downstream from $A$ to $B$ in $3$ $hrs$. If the speed of the boat in still water is $9$ $km/h$ and the speed of the current is $3$ $km/h$,the distance between $A$ and $B$ is......$km$.

From the given statements,decide which of the statement$(s)$ is/are necessary to answer the question.
What is the speed of a boat in still water?
$I.$ The boat covers $12 \text{ km}$ in $2 \text{ hours}$ downstream.
$II.$ The boat covers the same distance in $4 \text{ hours}$ upstream.
$III.$ The speed of the stream is $\frac{1}{3}$ that of the boat in still water.

$A$ boat covers $12\, km$ upstream and $18\, km$ downstream in $3\, hours,$ while it covers $36\, km$ upstream and $24\, km$ downstream in $6\frac{1}{2}\, hours.$ What is the speed of the current? (in $km/h$)

$A$ man can row at $10 \text{ km/h}$ in still water. If he takes a total of $5 \text{ h}$ to go to a place $24 \text{ km}$ away and return,then the speed of the water current is $... \text{ km/h}$.

$A$ man can row at $5 \, km/h$ in still water. If the velocity of the current is $1 \, km/h$ and it takes him $1 \, h$ to row to a place and come back,how far is the place? (in $km$)