A-B-C is a 40-120-20-degree triangle. D is a point on AC so that AD = BC. Find the measure of angle ADB. Only elementary geometry is allowed (no trigonometry).

See also: Poster of problem 1389.

Geometry Problems Ten problems: 1381-1390 Visual Index Open Problems All Problems Triangle Congruence 80-20-80 Isosceles Triangle View or Post a solution