Answer:
C. 70°
Step-by-step explanation:
but c = 180° - (2×20°) = 140°
b = 180° - c
b = 180° - 140° = 40°
[tex]{ \sf{a = \frac{1}{2}c }} \\ { \sf{a = ( \frac{1}{2} \times 140 \degree)}} \\ { \sf{a = 70 \degree}}[/tex]
