$A$ sailor goes $12$ $km$ downstream in $48$ $min$ and returns in $1$ $h$ $20$ $min.$ The speed of the sailor in still water is..........$km/h$.

If the speed of a swimmer in still water is $9\, km/h$,find the downstream speed of the swimmer when the river is flowing with a speed of $6\, km/h$ (in $km/h$).

$A$ steamer goes downstream from one port to another in $4\, h$. It covers the same distance upstream in $5\, h$. If the speed of the stream is $2\, km/h$,then find the distance between the two ports (in $km$).

$A$ boat running downstream covers a distance of $16\, km$ in $2\, hours,$ while for covering the same distance upstream,it takes $4\, hours.$ What is the speed of the boat in still water? (in $km/hr$)

$A$ boat covers a certain distance downstream in $2$ $hours$,but takes $4$ $hours$ to return upstream to the starting point. If the speed of the stream is $3$ $km/h$,find the speed of the boat in still water (in $km/h$).

$A$ 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 $km/h$.