Concept:Use the equal base angles of an isosceles triangle and the angle sum on a straight line.Explanation:The triangle is isosceles, so the two base angles are equal.One base angle is 50∘, hence the other base angle is a=50∘.The sum of interior angles of a triangle is 180∘.a+50∘+∠PRQ=180∘50∘+50∘+∠PRQ=180∘∠PRQ=180∘−100∘=80∘Angles on a straight line add up to 180∘.∠PRQ+b=180∘80∘+b=180∘b=180∘−80∘=100∘Answer:100∘, i.e. option D.
