SSC CHSL Model Paper 12

In AOB
AO = OB (radius)
∠OAB = ∠OBA (angles opposite to equal sides are equal)
∠OBA = 20°
∠OBC = ∠OCB = 30° (angles opposite to equal sides are equal)
∠ABC = 20 + 30 = 50°
∠AOC = 2 × 50° = 100° (angles subtended by a chord at the center is double the angle subtended at the circumference)