From Final Exam 2001
... ABC has angle A = 30 degrees, angle B = 60 degrees and angle C = 90 degrees. The angle at C is what is expected, for the exterior angle at C should be 180/2 = 90. ...
http://www.math.washington.edu/~king/coursedir/m444a03/notes/trans/12-01-rot-comp-ex-120-60.html