$A$ swimmer wants to cross a river which is flowing at a speed $v$. If the swimmer can swim in still water at speed $V$,the direction he should swim to cross the river in the least time is

$A$ glass windscreen,whose inclination with the vertical can be changed,is mounted on a car. The car moves horizontally with a speed of $2\,m/s$. At what angle $\alpha$ with the vertical should the windscreen be placed so that the rain drops falling vertically downwards with velocity $6\,m/s$ strike the windscreen perpendicularly?

$A$ motorboat is racing towards north at $25\; km/h$ and the water current in that region is $10\; km/h$ in the direction of $60^{\circ}$ east of south. Find the resultant velocity of the boat.

$A$ jet airplane travelling at the speed of $500 \; km \; h^{-1}$ ejects its products of combustion at the speed of $1500 \; km \; h^{-1}$ relative to the jet plane. What is the speed of the latter with respect to an observer on the ground?

Two trains,each $50 \, m$ long,are travelling in opposite directions with velocities $10 \, m/s$ and $15 \, m/s$. The time taken for them to cross each other is:

$A$ boy is running on a plane road with velocity $v$ with a long hollow tube in his hand. The water is falling vertically downwards with velocity $u$. At what angle to the vertical,he must incline the tube so that the water drops enter it without touching its sides?