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
   = $dis\tan ce during interval= area under the triangle=\frac{1}{2}\times 20\times 3 + 20\times 1$  = 30 + 20 = 50 m.