In a quadrilateral ABCD, the measure of the three angles A, B and C of the quadrilateral are 110°, 70° and 80° respectively. Find the measure of the third angle.

The measure of A = 110°
The measure of B = 70°
The measure of C = 80°
The sum of the four angles of the quadrilateral ABCD = ∠A + ∠B + ∠C +∠D=360°.
∠A + ∠B + ∠C = 110°+70°+80° = 260°
∠A + ∠B + ∠C +∠D = 360°
∠D = 360°-(∠A + ∠B + ∠C)
= 360°-260°
= 100°