The average speed is the ratio between the distance covered, and the time elapsed to cover that distance.
The total distance is the sum of the three distances covered:
[tex] d = 80km+50km+40km = 170km [/tex]
Similarly, the total time elapsed is
[tex] 1h + 0.5h + 0.25h = 1.75h[/tex]
So, althought you didn't always travelled with the same speed, you globally covered 170km in 1.75 hours. This leads to an average speed of
[tex] \dfrac{170\text{km}}{1.75\text{h}} \approx 97.14 \dfrac{\text{km}}{\text{h}} [/tex]
