maths~

OB = OC = radius
triangle OBC is an isosceles triangle
angle OBC = angle OCB = 180 - 120 = 60
angle BOC = 180 - angle OBC - angle OBC = 180 - 60 - 60 = 60
angle BAC = 0.5 x angle BOC = 0.5 x 60 = 30
(angle at circumference = 0.5 times of angle at centre)