Dots In A Circle

There are 40 dots spaced evenly in a circle. If 4 distinct dots $$W, X, Y$$ and $$Z$$ are picked randomly, what is the probability that $$WX$$ intersects $$YZ$$?

If the probability can be expressed as $$\dfrac ab$$, where $$a$$ and $$b$$ are coprime positive integers, find $$a+b$$.

×