You drive your car in a straight line at 15 m/s for 10 kilometers, then at 25

Question:

You drive your car in a straight line at 15 m/s for 10 kilometers, then at 25 m/s for another 10 kilometers.
(a) Is your average speed for the entire trip greater than, less than, or equal to 20 m/s?
(b) Choose the best explanation from among the following:
I. More time is spent at 15 m/s than at 25 m/s.
II. The average of 15 m/s and 25 m/s is 20 m/s.
III. Less time is spent at 15 m/s than at 25 m/s.