SAT Math XI

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?

NOTE:

A relay race is when one runner runs and then passes an item (usually a baton) to the other racer who is waiting on a certain point on the racetrack, and once received continues to run. A relay chase is pretty much the same thing, except they aren't passing batons but perhaps something else (more dangerous).