What is the average velocity and displacement of a body moving in a horizontal circle with constant speed?

A train is moving with a velocity of $90 \mathrm{km}/\mathrm{h}$. When the driver applies brakes the train comes to halt in $10 seconds$ .

(a) Calculate the retardation of the train.

(b) Distance covered by the train in $10$ seconds.                                                                                     

The brakes applied to a car produce a uniform retardation of $1.2 \mathrm{m}/{\mathrm{s}}^2$. Before stopping, the car covers a distance of $12 \mathrm{m}$. How long were the brakes applied? 

Which of the following are vector quantities?

Distance, speed, velocity, displacement, mass, acceleration.

Is it possible that average velocity of a body is zero but its average speed is not zero?

A body is moving with a velocity of $10 \mathrm{m}/\mathrm{s}$. If the motion is uniform, what will be the velocity after $20 \sec$ ?