Missing circle angle: DOF + AOB = 180 (half a circle). BOC and COD look fairly equal to me, which means that the equation would be 180 - 40 = BOC + COD
140 = BOC + COD
140/2 = 70
The answer for both would be 70 degrees.
Practice problems: keep in mind that a straight line = 180 degrees
1. D and E
2. D
3. A line with a square = 90 degrees, so 180 - 90 - 53 = g
G = 37 degrees
Hope this helps