Probability and set notation. Suppose that the probability that the Dow-Jones stock index increases today is $.54$, that it increases tomorrow is $.54$, and that it increases in both days is $.28$. What is the probability that it increases today but not tomorrow? I'm having trouble putting this in terms of set notation, can anyone help? Edit: fixed title and sentences.

If you draw a Venn diagram it becomes apparent that

$$\Pr(N\cap F^c) = \Pr(N) - \Pr(N\cap F) =.54-.28=.26,$$

where $N$ is today, $F$ is the future, and $^c$ their complements.