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Hi there!
∠C = 40° and triangle ABC is isosceles, therefore:
∠C = ∠A = 40°
Angles in a triangle sum up to 180°, so:
180 = 40 + 40 + ∠A
180 = 80 + ∠A
100 = ∠A
Thus, arc CB has a measure of 100°.
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