Ontario Curriculum Expectation:
6.E2.3 use the properties of supplementary angles, complementary angles, opposite angles, and interior and exterior angles to solve for unknown angle measures.
Three interior angles of a triangle add up to 180°.
Any quadrilaterals (a shape with 4 sides) can be made up with two triangles.
Since the three interior angles of a triangle adds up to 180°, all the interior angles of a quadrilateral adds up to 360°.
Finding the Unknown Angle
360° = a + b + c + d
360° = 100º + 85º + 90º + d
360° = 275º + d
d = 360° − 275º
d = 85º