Question 815229
Speed is distance per unit of time, in this case, miles per hour. So to solve the problem, you need to find the the total distance driven and divide that by the number of hours it took them to finish the trip. 
They drove 400 miles + 300 miles + 100 miles = 800 miles in
5 hours + 5 hours + 2 hours = 12 hours.
So to find their average speed, divide 800 miles by the total time, 12 hours, and you get 66.66 mph.
I hope this helps!