$A$ boat goes $6\, km$ upstream and back again to the starting point in $2\, hours$. If the velocity of the stream is $4\, km/hr$,what is the speed of the boat in still water? (in $km/hr$)

$A$ boat travels upstream from $B$ to $A$ and downstream from $A$ to $B$ in $3$ $hours$. 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........$km$.

$A$ swimmer covers a distance of $28\, km$ against the current and $40\, km$ in the direction of the current. If in each case he takes $4\, hours$,then the speed of the current 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 $km/h$).

$A$ boat travels $18 \, km$ upstream in $6 \, hours$. How long (in $hours$) will it take to cover the same distance downstream if the speed of the current is one-fourth the speed of the boat in still water?

$A$ boatman rows $1\, km$ in $5\, min$ along the stream and $6\, km$ in $1\, h$ against the stream. The speed of the stream is $km/h$.