$A$ 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 $km/h$).

$A$ 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..........$km/h$.

Two boats $A$ and $B$ start towards each other from two places,$108 \text{ km}$ apart. The speeds of boats $A$ and $B$ in still water are $12 \text{ km/h}$ and $15 \text{ km/h}$,respectively. If $A$ proceeds down the stream and $B$ proceeds up the stream,they will meet after .......... $\text{h}$.

The speed of a boat in still water is $6\, km/hr$ and the speed of the stream is $1.5\, km/hr$. $A$ man rows to a place at a distance of $22.5\, km$ and comes back to the starting point. The total time taken by him is

$A$ man can row $18 \ km/h$ in still water. It takes him thrice as long to row up as to row down the river. Find the rate of the stream. (in $km/h$)

If a boat goes upstream at a speed of $21\, km/h$ and comes back the same distance at $28\, km/h$,what is the average speed (in $km/h$) for the total journey?