Four points A_1, A_2, A_3, and A_4 are chosen at random on a circle. Find the probability that chords A_1 A_2 and A_3 A_4 intersect.
Out of four ways, there is only one way to set up the chords A_1 A_2 and A_3 A_4 so that they intersect, so the probability is 1/4.
