Answer:
6.5 MPH
Explanation:
Since he rode his skateboard for half an hour, you must multiply 2, that way he rode 1 hour. Since you multiply the 1/2 hour, you must also multiply 3 1/4, to get 6 1/2, or 6.5. Even though we know he didn't ride a full hour, if he had, and at the same rate, he would have rode 6.5 miles per hour.
Hope this helps! ;)