Answer:
(d) 60°
Step-by-step explanation:
Theorems
- Interior angles of a triangle sum to 180°
- Angles on a straight line sum to 180°
Therefore, the exterior angle of a triangle is equal to the sum of the two non-adjacent interior angles of the triangle.
⇒ (a + 20) + (a + 10) = (a + 90)
⇒ a + 20 + a + 10 = a + 90
⇒ 2a + 30 = a + 90
⇒ 2a + 30 - a = a + 90 - a
⇒ a + 30 = 90
⇒ a + 30 - 30 = 90 - 30
⇒ a = 60
