Check the picture below.
∡a has a vertical angle sibling of 40°, and vertical angles are always congruent.
∡b is the 3rd angle in a triangle, the other two are 40° and 90°, recall all interior angles in a triangle add up to 180°.
∡c is a linear angle, namely an angle on the same flat-line as another, and linear angles always add up to 180°.
