Question 366838
Let T = total amount of time of the trip.
Then during the first 2T/3 hours he averaged 120km/hr, and traveled (2T/3 hrs)*(120 km/hr) = 80T km. 
For the whole trip, he traveled (T hrs)*(100 km/hr) = 100T km.  Therefore in the last third of the trip he traveled 100T - 80T = 20T km.  The total time during the last third of the trip is T/3 hours.  Hence his average speed for the last third of the trip is (20T km)/(T/3 hrs) = 60 km/hr.