Let the distance travelled by car in one round be S km. Therefore total distance travelled by the is 2S km.
We know that:
V1=30 km/h, v2= 50 km/h
speed = distance /time
T1= S/30, T2 =S/50
∴total time= 8S/150
average speed = total distance/total time
Displacement=0 ∵because the car returns to the same point from where it had started.