Answer:
36 km/h
Explanation:
If d is the total distance, then the time spent driving at 20 km/h is:
t₁ = (d/3) / 20
t₁ = d / 60
And the time spent driving at 60 km/h is:
t₂ = (2d/3) / 60
t₂ = d / 90
So the total time is:
t = t₁ + t₂
t = d/60 + d/90
t = d/36
The average speed is the total distance divided by total time:
s = d / t
s = d / (d/36)
s = 36
The average speed is 36 km/h.
