$A$ boy walks to his school at a distance of $6\, km$ with a constant speed of $2.5\, km/h$ and walks back with a constant speed of $4\, km/h$. His average speed for the round trip,expressed in $km/h$,is:

$A$ car moves for half of its time at $80 \, km/h$ and for the rest half of the time at $40 \, km/h$. The total distance covered is $60 \, km$. What is the average speed of the car in $km/h$?

Write the relation between speed and the magnitude of velocity.

The position of an object moving along the $x$-axis is given by $x = a + b t^2$,where $a = 8.5 \; m$,$b = 2.5 \; m s^{-2}$,and $t$ is measured in seconds. What is the average velocity between $t = 2.0 \; s$ and $t = 4.0 \; s$ (in $; m s^{-1}$)?

$A$ car moves from $A$ to $B$ with a speed of $30 \ km/h$ and from $B$ to $A$ with a speed of $20 \ km/h$. What is the average speed of the car (in $km/h$)?

$A$ car travels a distance of $x$ with speed $v_1$ and then the same distance $x$ with speed $v_2$ in the same direction. The average speed of the car is