The average speed over the entire trip was 60 mph, which must equal the distance traveled (240 miles) times the elapsed time. Dividing 60 mph into 240 miles gives us the elapsed time: (240 miles) / (60 mph) = 4 hours total.
Next: determine the length of time required to drive the first 120 miles at 50 mph. It is (120 miles) / (50 mph), or 2.4 hours.
Next, find the length of time required to drive the second 120 miles. It is 4 hours less 2.4 hours, or 1.6 hours.
Lastly, find the average speed over which the second 120 miles were covered in 1.6 hours:
average speed (2nd 120 miles) = (120 miles) / (1.6 hours) = 75 mph.
