How do you find average velocity (vaverage) from acceleration (a) and time (t)?

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.