If you pick, at random, a line which passes through the circle, what is the probability that the section of your line that lies within the circle will be longer than the sides of the equilateral triangle?
The three red segments that make up the triangle are, of course, the same length as the sides of the triangle.
If you need some more visual input to work out what I mean, here are three random lines you could choose:
For those who are frightened of probability and statistics, you might want to try the simpler question: are there more shorter lines, more longer lines or most equally long lines?
It's worth trying to figure it out yourself before looking at the answer - here.