A driver sets out on a journey. for the first half of the distance she drives at the leisurely pace of 30 mi/h; during the second half she drives 60 mi/h. what is her average speed on this trip?

The average speed of a moving object is the rate of change of a certain distance with respect with time. It is equal to the total distance that was traveled by the object over the total time it takes to travel that distance. For this problem we need to assume that the total distance that was traveled would be equal to 120 miles. So, for the first half of the distance or 60 miles at a speed of 30 miles per hour, the time taken would be two hours. For the remaining 60 miles at a speed of 60 miles per hour, 1 hour is total time traveled. So, we calculate the average speed as follows:

Average speed = total distance / total time

Average speed = 120 miles / 2 hr + 1 hr

Average speed = 40 mi / hr