The triangle above is an isosceles triangle. It has two of its sides equal. Angles at the base of Isosceles triangle are the same. a=50∘ Sum of angles in a triangle is 180∘ a+50+ angle PRQ=180 a=50 50+50+ angle PRQ=180 100+ angle PRQ=180 Angle PRQ=180−100 Angle PRQ=80 The sum of angles on a straight line is 180∘ Angle PRQ+b=180 But angle PRQ=80 80+b=180 b=180−80 b=100