In trapezoid ABCD, sides AB and CD are parallel, angle A = angle D, and angle C = 3 times angle B. Find angle B.

Trapezoid ABCD is the right trapezoid because its two angles are the right angles.

∠**A = ∠D = 90°**

**∠B : ∠C = 1 : 3**

**∠B = 1/4 * 180° = 45°**

**∠C = 3/4 * 180° = 135° **