ABCD is a cyclic quadrilateral

∴ ∠BCD + ∠BAD = 180°[Opposite angle of a cyclic quadrilateral are supplementary]

⇒ 100° + ∠BAD = 180°

⇒ ∠BAD = 80°

In triangle ABD,

∠ADB + ∠ABD + ∠BAD = 180°[Sum of angles of triangle]

⇒ ∠ADB + 50° + 80° = 180°

⇒ ∠ADB + 130° = 180°

⇒ ∠ADB = 50°