If the average velocity of a body is equal to the mean of its initial velocity and final velocity,then the acceleration of the body is

Give examples to distinguish:
$(i)$ Distance and displacement.
$(ii)$ Speed and velocity.
$(iii)$ Acceleration and retardation.

$A$ train starting from rest picks up a speed of $10\, m s^{-1}$ in $100\, s$. It continues to move at the same speed for the next $250\, s$. It is then brought to rest in the next $50\, s$. Plot a speed-time graph for the entire motion of the train and calculate:
$(i)$ acceleration of the train while accelerating,
$(ii)$ retardation of the train while retarding,
$(iii)$ the total distance covered by the train.

$A$ hiker rides $700\, m$ north,$300\, m$ east,$400\, m$ north,$600\, m$ west,$1200\, m$ south,$300\, m$ east,and finally $100\, m$ north. Draw the path of motion of the hiker. What distance did he cover? What was his displacement?

$(i)$ What can be depicted from the graph regarding the motion of the object?
$(ii)$ Find the value of acceleration from the graph.

$A$ motorcyclist drives from $A$ to $B$ with a uniform speed of $30\, km\, h^{-1}$ and returns back with a speed of $20\, km\, h^{-1}$. Find its average speed (in $km\, h^{-1}$).