On a 20-mile bike ride, you ride

On a 20-mile bike ride, you ride the first 10 miles at an average speed of 8 mi/b. What must your average speed over the next 10 miles be to have your average speed for the total 20 miles be

(a) 4 mi/b?
(b) 12 mi/b?

(c) Given this average speed for the first 10 miles, can you possibly attain an average speed of 16 mi/b for the total 20-mile ride? Explain.