Sonic and Flash each run at a constant rate of 7.5 miles per hour. Road Runner runs at a constant rate of 10 miles per hour. Speedy Gonzales runs at a constant rate of 12 miles per hour. In a relay chase with these four runners running one right after the other, Sonic runs 0.3 miles, then Flash runs 0.3 miles, then Road Runner runs 0.5 miles, then Speedy Gonzales runs 0.24 miles. What is their average speed in miles per hour?
