Answer: 15m/s
Step-by-step explanation: Average Velocity is vector describing the total displacement of an object and the time taken to change its position. It is represented as:

At t₁ = 1.0s, displacement x₁ is:

x(1) = 28
At t₂ = 4.0s:

x(4) = 73
Then, average speed is

v = 15
The average velocity of a car between t₁ = 1s and t₂ = 4s is 15m/s