Given:
A bus traveled on a level road for 6 hours at an average speed of 20 miles per hour .
The distance is calculated as,

The distance covered by bus on level road is faster than it raveled on a winding road.
The time spent on the winding road was 2 hour. So, the distance is,

The total distance was 360 miles.

Answer: the average speed on the level road is 120 mph