The velocity-time graph shows the motion of an electric vehicle during a 50-second test run on a straight track.
During the test run, the vehicle accelerates from rest to a maximum velocity of 36 m/s in 15 s, decelerates uniformly to 12 m/s over the next 10 s, and then travels at a constant velocity of 12 m/s for the remaining 25 s.
What is the average speed of the vehicle over the entire 50-second journey?
13.8 m/s13.8\text{ m/s}13.8 m/s
16.2 m/s16.2\text{ m/s}16.2 m/s
18.0 m/s18.0\text{ m/s}18.0 m/s
21.6 m/s21.6\text{ m/s}21.6 m/s