In an isosceles triangle, the base angles are congruent. The third angle is called the vertex angle.
Here, the vertex angle is <A.
Therefore, m<C = m<B.
m<A = 3m<B + 20
m<A + m<B + m<C = 180
3m<B + 20 + m<B + m<B = 180
5m<B + 20 = 180
5m<B = 160
m<B = 32
m<C = m<B = 32
Answer: m<C = 32 deg