Chen rode his stake board 3 3/4 miles of an hour. What was his average speed in miles per hour?

For one hour he rode his skateboard for 3.75mph

3 3/4 miles as a decimal is 3.75 miles. He rode for one hour. Average speed within an interval is (distance made) / (interval duration). So the average speed = 3.75 miles / 1 hour, or 3.75 mph.