the graph is velocity over time, and they only want you to look between 0s and 10s.
0-5s, your AVERAGE velocity is 10m/s (start at 20, end at 0). 10m/s*5s=50m
5-10s, your AVERAGE velocity is -10m/s (start at 0, end at -20. -20m/s*5s=-50m
they ask for DISTANCE total, not your displacement, so you add magnitudes, so the answer is 100m. displacement is a path independent question, looking only at start and end points. distance is not.