$A$ man can row $15\, km/h$ downstream and $9\, km/h$ upstream. The speed of the boat in still water is.....$km/h$

$A$ 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 $km/hr$)

$A$ boatman can row $48\, km$ downstream in $4\, hours$. If the speed of the current is $5\, km/h$,then find in what time will he be able to cover $8\, km$ upstream? (in $hours$)

$A$ swimmer's speed in downstream is $11 \, km/h$ and speed of the stream is $1.5 \, km/h$. Find the upstream speed of the swimmer? (in $km/h$)

$A$ 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:

$A$ man takes $3$ $hours$ $45$ $minutes$ to row a boat $15 \; km$ downstream of a river and $2$ $hours$ $30$ $minutes$ to cover a distance of $5 \; km$ upstream. Find the speed of the river current in $km/hr$.