$A$ boat goes $48\, km$ downstream in $20\, h$. It takes $4\, h$ more to cover the same distance against the stream. What is the speed of the boat in still water? (in $km/h$)

$A$ boat goes $40\, km$ upstream in $8\, h$ and $36\, km$ downstream in $6\, h$. The speed of the boat in still water is......$km/h$

In a stream running at $2 \, km/h$,a motorboat goes $10 \, km$ upstream and returns to the starting point in $55 \, min$. Find the speed of the motorboat in still water (in $km/h$).

$A$ 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:

$A$ boat running downstream covers a distance of $20\, km$ in $2\, h$ while it covers the same distance upstream in $5\, h$. Then,the speed of the boat in still water is ....... $km/h$.

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.