Answer + Explanation:
16.
sum of opposite two angles = one exterior angle
50° + 20° = d
d = 70°
17.
A straight line has total 180° angle.
a + d = 180°
90° + d = 180°
d = 180° - 90°
d = 90°
18.
sum of opposite two angles = one exterior angle
52° + 78° = d
d = 130°
19/20.
If a = 90° then d = 90°
Then b = 90 - c and c = 90 - b
21.
If d = 105°, then c = 105 - b