The speed of a swimmer is $4\,km\,h ^{-1}$ in still water. If the swimmer makes his strokes normal to the flow of river of width $1\,km$, he reaches a point $750 \,m$ down the stream on the opposite bank.The speed of the river water is $.........km h ^{-1}$

$A$ ship $X$ moving due north with speed $v$ observes that another ship $Y$ is moving due west with same speed $v$. The actual velocity of $Y$ is $........$.

A bomb is dropped by fighter plane flying horizontally. To an observer sitting in the plane, the trajectory of the bomb is a :

A boat covers certain distance between two spots in a river taking $t_1$ hrs going downstream and $t_2$ hrs going upstream. What time will be taken by boat to cover same distance in still water?

A boat crossing a river moves with a velocity $v$ relative to still water. The river is flowing with a velocity $v / 2$ with respect to the bank. The angle with respect to the flow direction with which the boat should move to minimize the drift is

A river is flowing with velocity $5\ km/hr$ as shown in the figure. A boat starts from $A$ and reaches the other bank by covering shortest possible distance . Velocity of boat in still water is $3\ km/ hr$. The distance boat covers is    ......... $m$