Velocity = (displacement) / (time)
'Displacement' is the distance between the start-point and end-point,
so the car's displacement is zero, and its average velocity is zero.
However, Speed = (distance covered) / (time to cover the distance).
The car covered a distance of 30 km in 15 minutes,
so its average speed is
(30km) / (0.25 hour) = 120 km/hr.
'Velocity' is not simply a word you use for 'speed' when
you want to sound smarter. They're two different things.
