Hello,
Average speed is total distance divided by total time. From the problem, our total distance is given as 500 kilometers and given time is 5 hours. Therefore, the average speed is:

Therefore, the average speed is 100 km/h. Please let me know if you have any questions!
Answer:
the third stage was 480 km long
Explanation:
Stage 1:
Time = 1 hours
Speed = 80km
Stage 2:
Time = 2 hours
Speed = 200km
Stage 3:
Time = 4 hours
Let the Distance at the stage 3 be x
Average speed of the train route = 100 km/h
So


Lets find the speed at stage 1
Speed = 
Speed = 
Speed 1= 80 km/hr
The speed at stage 2
Speed = 
Speed = 
Speed 2 = 100 km/hr
The speed at stage 3
Speed = 
Speed = 
Speed 3 = 
we kow that average is ,









x = 480
I think it’s R=U/I = 120/5 = 24