Velocity-time (v-t) graph for a moving object is shown in the figure. Total displacement of the object during the time interval when there is non-zero acceleration and retardation is

Between time interval 20 sec to 40 sec, there is non-zero acceleration and    retardation.  Hence distance travelled during this interval  = Area between time interval 20 sec to 40 sec   =area of triangle + area of rectangle , below the triangle   = distance during interval= area under the triangle=12×20×3   +  20×1  = 30 + 20 = 50 m.