Answer:
1) Average velocities:
Given the magnitudes of 2 velocity vectors, in different time instants, the average velocity is defined as the difference of the 2 velocity magnitudes, in absolute value, divided by the interval of time occuring between the measurement of the velocity magnitudes.
2) Average acceleration
Given the magnitudes of 2 acceleration vectors, in different time instants, the average acceleration is defined as the difference of the 2 acceleration magnitudes, in absolute value, divided by the interval of time occuring between the measurement of the acceleration magnitudes.
As you see the definitions are pretty similar.
Note: I didn't copy-paste this answers from any other site.
