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° 
