In the figure, <4 is an exterior angle to VAUL

https://snag.gy/MsEfJc.jpg

Explain why m<4 is equal to the sum of the measures of the two nonadjacent interior angles.

The sum of angle 3 + angle 4 = 180

The sum of the interior angles of the triangle = 180

So

angle 3 + angle 4 = angle 3 + 55 + 20 subtract angle 3 from both sides

angle 4 = 55 + 20 = sum of the measures of the two non-adjacent interior angles