Given: ∠BAO = 60°

Since, OA and OB are radius of the circle, therefore OA = OB

⇒ ∠ ABO = ∠BAO = 60° (∵ given that ∠BAO = 60°)

Since angle in the same segment are equal, therefore

∠ABC = ∠ADC (angles in the same segment AC are equal)

∠ABC = ∠ABO = 60°

∴ ∠ADC = 60°