Four points P, Q, R, and S are chosen at random on the circumference of a circle. Find the probability that chord PQ and chord RS intersect.
Out of 4 ways, there is only one way to set up the chords so that PQ and RS intersect so the probability is 1/4.
"Anchor" P at any place on the circle
Now....going clock-wise.....we could have the following sequences of points
QRS
QSR
RQS
RSQ
SQR
SRQ
Notice that only 2 of these are the intersecting chords PQ and RS
So....the probability is 2/6 = 1/3
+232 
