The given triangle ABC is an obtuse triangle because one of its angles it greater than 90 degrees. The sum of the angles in a triangle is 180 degrees. This means that
Angle A + Angle B + Angle C = 180 degrees
Looking at the triangle,
Angle A = 30 degrees
Angle C = 20 degrees
Therefore,
30 + angle B + 20 = 180
Angle B + 50 = 180
Subtracting 50 from the left hand side and the right hand side of the equation, it becomes
