In Δ OAB,
OA = OB [both are the radius of a circle]
∠OAB=∠OBA⇒∠OBA=40∘ [angles opposite to equal sides are equal]
Also, ∠AOB+∠OBA+∠OAB=180∘ [by angles sum property of a triangle]
We know that, in a circle, the angle subtended by an arc at the centre is twice the angle subtended by it at the remaining part of the circle.