Answer/Step-by-step explanation:
When twi straight lines intersect, they form 2 pairs of opposite vertical angles that are congruent.
In the diagram given, the following pairs of vertical angles are formed:
<c and 40°
<b and <d
<c = 40° (vertical angles are congruent)
<b and 40° are linear pairs. They are both supplementary. Therefore:
<b = 180° - 40
<b = 140°
<d = <b (vertical angles)
<d = 140° (substitution)