How do you work out the average speed?

That depends on that data you have.

If you have a start and finish time, and the overall distance then you divide the distance in miles (or other units) by the number of hours and you'll get miles per hour.

If I leave home at 07:15 and get to work at 9:00, having travelled 6.5 miles then I have an average speed of 6.5 / 1.75 = 3.71 mph. (but that's enough about the Dublin bus service!)If you have two or more different speeds, and the length of time each was travelled then you wan work out the overall distance and the overall time, and proceed as above.