Acar accelerates from rest to a velocity of 5 meters/second in 4 seconds. what is its average acceleration over this period of time?

In this question, due importance should be given to all the information's that are provided in the question. Based on those information's the answer to the question can be easily deduced. Let us first write down all the provided information's.
Initial Velocity (V1) = 0 m/s
Final Velocity (V2) = 5 m/s
Initial time (T1) = 0 sec
Final Time (T2) = 4 sec
Then
Average acceleration of the car = (V2 -V1)/(T2 - T1)
                                                   = (5 - 0)/(4 - 0) m/s^2
                                                   = 5/4 m/s^2
                                                   1.25 m/s^2
So the average acceleration of the car over this period of time is 1.25 m/s^2. I hope the process is clear for you to understand.

|Average acceleration| = (change in speed) / (time for the change)

Change in speed = (speed at the end) - (speed at the start)

|Average acceleration| = (5 m/s - 0) / (4 sec)

                                       =   (5 / 4)  m/s²

                                       =      1.25 m/s²  .