The formula for average speed is

So we can just substitute our data.

- its the result
Answer:
v=9.6 km/s
Explanation:
Given that
The mass of the car = m
The mass of the truck = 4 m
The velocity of the truck ,u= 12 km/s
The final velocity when they stick = v
If there is no any external force on the system then the total linear momentum of the system will be conserve.
Pi = Pf
m x 0 + 4 m x 12 = (m + 4 m) x v
0 + 48 m = 5 m v
5 v = 48

v=9.6 km/s
Therefore the final velocity will be 9.6 km/s.
Answer:
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