Why is -20pi/3 conteminal with 4pi/3

Answer 1

Perhaps you can see it better if we change it to degrees
-20π/3 radians = -1200°
This would be 3 complete clockwise rotations, ending up at -120°
We can also reach -120° by going counter-clockwise for 240°

Well, and how about that?
Isn't 4pi/3 radians = 240°