Question

If ABC is an isosceles triangle with base , what is the measure of C?

Answer 1

Angle B is supplementary to the external angle of 60 degrees, so B = 120.
Since A+B+C = 180 for a triangle,
A+C = 180 - 120 = 60
And for an isosceles triangle, A=C, so
C+C = 60
C = 30
The answer is 30 degrees.