Morwen bikes around his neighborhood for an hour. He averages 15 mph for the first 40 minutes, but then he tires and averages 10 mph for the last 20 minutes. What is is his average speed for the whole hour?
d = 15mi/h*(40/60)h + 10mi/h*(20/60)h = 13.33 miles. - Total dist. traveled.
r = 13.33mi/1h = 13.33 mi/h.