a) The speed remained constant.
If the line on a distance-time graph has a constant gradient then the speed is constant.
This can be read off the graph: after 20 s the car had travelled 200m, after 60 s it had traveled 800m So the distance travelled is 800- 200 = 600 m.
c) 15 m/s
Speed = distance/time, v=600/40 = 15 m/s. Don’t forget the units.
d) It stopped
The line is horizontal during this time, indicating that the car was not moving (no distance was travelled).