Geometry Problem 1389: Triangle, 40-120-20 degree, Angle, Congruence

Problem

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).

 

Geometry Problem 1389: Triangle, 40-120-20 degree, Angle, Congruence, Tutoring

 

See also:
Poster of problem 1389.