For the following velocity-time graph, the average speed for the motion during first 80 seconds is 

 

Total displacement in 80s is zero, where as total distance travelled is sum of the magnitudes of areas under the curve  for 0 to 40s and 40s to 80s. 

Total distance = 1/2 x 40 x 10 + 1/2 x 40 x 10

                           = 400m

Average speed = 400m/80s

                            = 5m/s

The average velocity is zero.