Geometry Essentials · Superchallenge problem

Twelve rays from a point $O$ divide the full turn around $O$ into twelve $30^\circ$ angles. Three of the twelve rays are chosen at random. Each two of the chosen rays form an angle (if two chosen rays are opposite, their angle is a straight angle, $180^\circ$). What is the probability that one of these three angles equals the sum of the other two?