Given:
∠QTP = 100°
Property 1: The tangent at a point on a circle is at right angles to the radius obtained by joining center and the point of tangency.
Property 2: Sum of all angles of a quadrilateral = 360°.
By property 1,
∠OPT = 90° and ∠OQT = 90°
And,
By property 2,
∠QTP + ∠OPT + ∠OQT + ∠POQ = 360°
⇒ ∠POQ = 360° – ∠QTP + ∠OPT + ∠OQT
⇒ ∠POQ = 360° – 100° + 90° + 90°
⇒ ∠POQ = 360° – 280°
⇒ ∠POQ = 80°
Hence,
∠POQ = 80°