AMC10 2011 10A, Problem 20

Two points on the circumference of a circle of radius $r$ are selected independently and at random. From each point a chord of length $r$ is drawn in a clockwise direction. What is the probability that the two chords intersect?

Solution: Fix a point $A$ from which we draw a clockwise chord. In order for the clockwise chord from another point $B$ to intersect that of point $A$, $A$ and $B$ must be no more than $r$ units apart. By drawing the circle, we quickly see that $B$ can be on $\frac{120}{360}=\fbox{\frac{1}{3}}$ of the perimeter of the circle. (Imagine a regular hexagon inscribed in the circle)

Answer: \frac{1}{3}