A triangle's exterior angle is formed by extending one side. The two remote interior angles measure 50° and 65°. What is the measure of…

The exterior angle theorem says an exterior angle of a triangle equals the sum of the two remote (non-adjacent) interior angles. So the exterior angle is $50^\circ+65^\circ=115^\circ$, choice B. Check: the third interior angle is $180^\circ-50^\circ-65^\circ=65^\circ$, and its supplement is $180^\circ-65^\circ=115^\circ$. Choice A ($65^\circ$) is the adjacent interior angle, not the exterior one. Choice C ($130^\circ$) doubles the $65^\circ$ angle. Choice D ($180^\circ$) is the sum of all three interior angles, not an exterior angle.