Acar is driven for 2.0 h at 40.0 km/h, then for another 2.0 at 60 km/h in the same direction

Although I think... 
the average velocity means, total distance over total time 

so consider the formula speed=distance/time. 

And break it down, the first part: 
speed=40 km/h, time 2 h 
distance= speed *times* time 
= 40 * 2 
= 80 km 

Then the second part: 
speed=60 km/h, time 2 h 
distance= speed *times* time 
= 60 * 2 
= 120 km 

Total distance / Total time = Average Velocity (keep in mind the basic formula speed=distance/time) 
therefore, (120+80) km / 4 hrs 
= 50 km/h 

Hope I helped :)