Online Geometry Problem 2. Triangle, Angles, Auxiliary Lines, 20-80 Degrees. Level: High School, Honors Geometry, College, Mathematics Education

Prove the following: if, in a triangle ABC, angle BAO = angle OAC = 20°, angle ACO = 10°,and angle OCB = 30°, then angle OBC = 80°. Try to use elementary geometry (Euclid's Elements.)