A octagon is 1080 degrees. The half circles are showing which angles are the same. So all the red circles are 120 degrees. To find the green circles, multiply 120 by 4 for the total of the four angles and then subtract it from 1080:
120(4) = 480
1080 - 480 = 600
So 600 is the total of all the green half circles added together, so divide 600 with 4 because there are four green half circles.