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
 
        
             
        
        
        
Acceleration = velocity/ time 
Acceleration = 0.7-0.3 /30= 0.01 m/s^2 
Notice that velocity is calculated the final speed minus the initial !