Charlie flew from Phoenix to Tucson, a distance of 100 miles, at a constant speed of 100 mph. He then returned at a constant speed of 50. mph. What was his average speed?

66.7 mph

Explanation:

Average speed = total distance / total time

We know the total distance is 200 miles. We need to find the total time.

During the first trip:

t = 100 mi / 100 mph

t = 1 h

During the second trip:

t = 100 mi / 50 mph

t = 2 h

So the average speed is:

s = 200 mi / 3 h

s = 66.7 mph

Round as needed.