In the adjoining figure, A, B, and C are three points on a circle O. If angle AOB = 90° and angle BOC = 120°, then angle ABC is:
75°
angle AOC = 360° - (angle AOB + angle BOC)
angle AOC = 360° - 90° + 120° = 150°
therefore angle ABC = 1/2 × angle AOC = 75°
[because, The angle subtended by a chord at the centre of a circle is twice the angle subtended by the same chord at any other point on the remaining part of the circle]