A car initially at rest accelerates uniformly for ten seconds, reaching a final speed of 12 m/s . It then accelerates uniformly again for ten…

A car initially at rest accelerates uniformly for ten seconds, reaching a final speed of 12 m/s . It then accelerates uniformly again for ten seconds, reaching a final speed of 30 m/s. During this 20-second period, the average speed (in m/s) of the car is :