Let X be a discrete random variable. If Pr(X<5) = 2/9, and Pr(X<=5) = 5/18, then what is Pr(X=5)?

Here's a clue:

The range given by the second subset (X<=5) is actually the same as the range given by the first subset (X<5) and the target (x=5)

It's actually quite simple, and there is quite a lot of help out there on the net.