Q.A car travels at a speed of 50 km/hr for 30 minutes, at 30 km/hr for the next 15 minutes and then at 70 km/hr for the next 45 minutes. What is the average speed of the car?

Average speed over a time interval is the total path length covered divided by that interval — a scalar, since path length has no direction. Instantaneous speed is the limiting value of average speed as the interval shrinks to zero around a given instant; in that limit, the (infinitesimally short) path length covered becomes exactly equal to the magnitude of the (infinitesimally short) displacement, so instantaneous speed is always precisely equal to the magnitude of the instantaneous velocity — a relationship that does NOT hold, in general, for the corresponding average quantities.

When the motion is entirely in one direction (no reversals), average speed equals the magnitude of average velocity. As soon as the object reverses direction at any point during the interval being averaged, the path length exceeds the magnitude of the net displacement, so the average speed becomes strictly larger than the magnitude of the average velocity — a classic exam point, most commonly illustrated with a round trip where the average velocity's magnitude can even fall to zero while the average speed stays clearly positive.