On a 20 mile bike ride, you ride the first 10 miles at an average speed of 8 mi/hr. Determine your average speed for the next 10 miles so that your average speed for the entire 20 miles will be (a) 4 mi/hr (b) 10 mi/hr?

What have you figured out? Distance =Speed x Time. How long did it take to ride the first 10 miles S=D/T, T=D/S