$A$ man can row upstream $36 \text{ km}$ in $6 \text{ hours}$. If the speed of a man in still water is $8 \text{ km/hr}$,find how much distance he can cover downstream in $10 \text{ hours}$? (in $\text{km}$)

$A$ river is flowing at a speed of $5\, km/h$ in a particular direction. $A$ man,who can swim at a speed of $20\, km/h$ in still water,starts swimming along the direction of flow of the river from point $A$ and reaches another point $B$ which is at a distance of $30\, km$ from the starting point $A$. On reaching point $B$,the man turns back and starts swimming against the direction of flow of the river and stops after reaching point $A$. The total time taken by the man to complete his journey is:

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?

$A$ man rows a boat $18 \ km$ in $4 \ h$ downstream and returns upstream in $12 \ h$. The speed of the stream (in $km/h$) is

$A$ 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?

$A$ man can row at a speed of $15\, km/hr$ in still water to a certain upstream point and back to the starting point in a river which flows at $3\, km/hr$. Find his average speed for the total journey (in $km/hr$).