IXL- Missing Angle

This IXL math is found in Year 8, U.7. It is about a missing angle in triangles and quadrilaterals (4 sided shapes). To figure out the missing angle you add all the current angles together. Example- 66+142+43=251. The sum of all the angle numbers must be 360 degrees. That only applies to a quadrilateral. For a triangle, the sum must be 180 degrees. Then you subtract the sum of all the current numbers by 360 or 180, depending on the shape. Example- 360-251-109.