When is the relative velocity of two moving cars greater than the individual velocity of both cars?

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?

When is the relative velocity of two cars moving with unequal velocities less than the individual velocity of either car?

$A$ man crosses a $320 \ m$ wide river perpendicular to the current in $4 \ minutes$. If in still water he can swim with a speed $5/3$ times that of the current,then the speed of the current,in $m/min$ is

$A$ man goes $10\,km$ downstream in $2\,hrs$ and $30\,km$ upstream in $10\,hrs$. How much time will he take to swim $40\,km$ in still water?