Concept:The angle at the centre is twice the angle at the circumference subtending the same arc.Angles around a point sum to 360∘.Explanation:Let α be the angle at the centre that subtends the same arc as ∠XWZ=m∘.By the circle theorem:α=2m∘The two central angles, α and ∠XOZ=10m∘, together make a full turn around O.α+10m∘=360∘Substitute α=2m∘:2m∘+10m∘=360∘12m∘=360∘m=12360=30