OENote that there are four angles in the fgure: the three interior angles of triangle ABC and the exterior angle at vertex A. Since the measure of the exterior angle at vertex A is x degrees, it follows that the measure of interior angle A is (180 - x)°. Also, since AB = BC, it follows that triangle ABC is isosceles and the measures of interior angles A and C are equal. Terefore the measure of interior angle C is also (180 - x)°. Since the sum of the measures of the interior angles of a triangle is 180° and you are given that the measure of interior angle B is 40°, it follows that \(40 + (180 - x) + (180 - x) = 180.\) Solving the equation for x gives x = 110. Since Quantity B is 120, the correct answer is Choice B.
Answer: B