Explanation:

When a BODY is travelling with a non-uniform velocity we calculate the instantaneous velocity meaning the velocity of the body at any instant during it's TRAVEL. Instantaneous velocity is given by :

the distance travelled by the body in a very short time INTERVAL / time interval

The time should be chosen keeping in mind that the direction of motion of the body does not change and time interval is so short that the body appears to MOVE in one dimensional motion.

Average speed is the average rate of of change of distance with respect to time.it is used when a body is MOVING with non-uniform speed

avg. speed = total distance / total time or ( initial speed + final speed )/2

Average velocity is the average rate of change of displacement with time. it is used when a body traverses with a non-uniform velocity.

avg. velocity = total displacement / total time or ( initial velocity + final velocity) /2

Avg. velocity of a body can be zero even if the avg. speed is not zero. this happens when the total displacement of the body is zero

Avg. speed = avg. velocity if the body travells in one dimensional motion,i.e., takes the shortest distance between two points

avg. velocity can never be greater the avg. speed.