Inscribed angle, find the degree measure of the angle ABC (O-center of the circle) if the angle is ADC = 50 degrees.

The angles ADC and ABC are inscribed and rest on the same arc, since the vertices of the corners coincide.

Two inscribed angles resting on the same arc are equal (a consequence of the inscribed angle theorem).

Since the angle ADC = 50 °, and the angles ABC and ADC are based on the same arc AC, then the angle ADC = ABC = 50 °.

Answer: ABC = 50 °.