SPEED is
(distance covered) / (time to cover the distance) .
The car's average SPEED is
(100 km) / (15min)
= 6-2/3 km per minute.
VELOCITY is
(distance and direction between start-point and end-point)
divided by
(time to travel from start-point to end-point).
The car ended up at the starting point, so the distance between
stating-point and end-point is zero.
Velocity = (zero) / (time to travel from start-point to end-point)
= Zero .