In the following figure, O is the center of the circle. Find the value of b.

Mark the point of intersection of AC and BD as E:

Since, AC is a straight line.

∴ ∠AEB + ∠BEC = 180°

⇒ 120° + ∠BEC = 180°

⇒ ∠BEC = 180° - 120° = 60°.

In △BEC,

⇒ ∠BEC + ∠ECB + ∠CBE = 180°

⇒ 60° + ∠ECB + 25° = 180°

⇒ ∠ECB + 85° = 180°

⇒ ∠ECB = 180° - 85° = 95°.

We know that,

Angles subtended by the same chord on the circle are equal.

∴ b = ∠ACB

From figure,

∠ACB = ∠ECB = 95°.

Hence, b = 95°.