Given,
Velocity for one third of distance V1=20km/h
Velocity for two third of distance V2=60km/h
Time take to cover one third of distance t1=d/3V1=d/320
Time taken to cover two third of distance t2=2d/3V2=2d/360
Average velocity =Totaldistancetotaltime
Vav=dt1+t2=dd/320+2d/360=36km/h