Question

Find the value of x.

Answer 1

(4x+5) + (3x+20) = 8x+15
[Reason - Exterior angle is equal to sum of its two interior angles]

7x + 25 = 8x + 15
x = 10

Answer 2

An exterior angle of a triangle is equal to the sum of the opposite interior angles, so:

8x + 15 = 3x + 20 + 4x + 5
8x + 15 = 7x + 25
8x - 7x = 25 - 15
x = 10