Question

How do you find the average velocity over an interval?

Answer 1

If the moving object is undergoing constant acceleration, then

$v_{\rm avg}=\dfrac{v_i+v_f}2$

(take the average of the initial and final velocities, if they're given)

If you don't know the acceleration or initial/final velocities, then you have to resort to the usual definition,

$v_{\rm avg}=\dfrac{\Delta x}{\Delta t}$

where $\Delta x$ is the change in position and $\Delta t$ is the duration of the interval.