Answer:
60°
Step-by-step explanation:
sum of interior angles of quadrilateral is 360°
∠C = 70°
∠D = 50°
∠C + ∠D = 120°
∴ ∠A + ∠B = 360° - 120° =240°
AO and BO bisect ∠A and ∠B
∠OAB + ∠OBA = 1/2 x ( ∠A + ∠B) = 120°
∠AOB = 180° - (∠OAB + ∠OBA) = 180° - 120° = 60°
