20 apartments on 4 floors = apartments per floor
2 answers:
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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°.
