You're right that the average speed is 15 mph. As to "what does this mean," well, this is the average speed. You're not going to be traveling at exactly 15 mph for the entire trip. Right when you start and stop the trip, the speed will be close to 0 mph, and sometimes you'll be going over 15 mph.
The total distance is still 10 miles. The total time it took was 46 minutes, or 23/30 hr.2. On the return trip from the beach, you pedal hard for the first ten minutes and cover 4 miles. Tired, you slow down and cover the last 6 miles in 36 minutes. What is your average speed for the return trip?
I got 5/23 miles/min. Is this correct?
which is 5/23 miles/min. (However, you should use the same units, mph, as in the first part, so you should state the answer as 13.04 mph.)