Answer:
40 km/h
Explanation:
The fastest speed would be the most distance traveled in less time. This would be the steepest part of the line. So from hours 2 to 4.
This means they traveled 80 kilometers (100-20) in two hours.
This is 80 kilometers per 2 hours. To simplify, we divide each by 2 getting 40 kilometers per hour.
Hope this makes sense