Physic C....series of systems of particles?

Question

a 1000 kg automobile is at rest at a traffic signal. At the instant the light turns green, the automobile starts to move with a constant acceleration of 4.0 m/s^2. At the same instant a 2000kg truck, traveling at a constant speed of 8.0 m/s, overtakes and passes the automobile

a) How far is the center of mass of the automobile-truck system from the traffic light at t = 3.0s?

b) What is the speed of the center of mass of the automobile-truck system then?

Answer 1

t =  3 sec

1000 kg car

constant acceleration of 4.0 m/s^2

displacement =  .5 * 4 * 3^2 = 18 m

2000kg truck

constant speed of 8.0 m/s

displacement =  8 * 3 = 24 m

So, after 3 seconds they are 6 m apart. ( 24 -18 = 6)

The center of gravity is closer to the more massive object.

So,

a) the center of mass is at 18 +4 and 24-2 or 22 m

The car traveled 18 m in 3 sec so must be going an average speed of 6 m/s

The truck is going 8 m/s

So, I guess,

b) the average speed of the system is (8+6)/2 = 7 m/s

Not convinced the b) answer is right.