Answer: Choice D) 30
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How I got this answer:
Use the exterior angle theorem. This theorem says adding any two remote interior angles of a triangle is the sum equal to the exterior angle.
The interior angles x and 70 add to x+70, which is equal to the exterior angle 100
x+70 = 100
x+70-70 = 100-70
x = 30
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Another route is to mark y as the angle adjacent to 100, and this angle y is inside the triangle. It is the third missing angle. Solve 100+y = 180 for y to get y = 80
The triangle has interior angles 80, 70 and x. They add to 180 leading to
80+70+x = 180
x+150 = 180
x+150-150 = 180-150
x = 30
