The equation to work out the speed is distance/time.
If we apply this equation to your numbers we will first need to find the total distance travelled.
80km/h for an hour means you travel 80km
40km/h for 2 hours means you will travel 80km
80+80 is 160km in total.
The time taken to complete this distance is 3 hours so, applying the equation we get
speed = 160/3 = 52.33km/h
