Velocity is calculated as distance over time, so the average velocity over an interval will be the total distance traveled in that interval divided by the total time taken to travel that distance.
Consider a car whose position, s, is given by the table
t (sec) 0 0.2 0.4 0.6 0.8 1
s (ft) 0 0.5 1.7 3.8 6.5 9.6
Estimate the velocity at t=0.2
Sketch a graph of the data then sketch a smooth curve connecting those points. The instantaneous speed at 0.2 s will be the slope of the line that is tangent to your curve at 0.2 s.