probability of “or” If its not possible for two events to happen at the same time, they are called mutually exclusive:
\begin{equation} P(E\ or\ F) = P(E)+P(F) - P(E \cap F) \end{equation}
This is called the inclusion exclusion principle. This is what motivates inclusion exclusion counting. General inclusion exclusion principle Its scary. Think about this: We basically need to alternate between adding and subtracting. (i.e.: in our case here, we add all the odd-group pairs (for P(x) and P(xyz)), we subtract the even-number pairs (for p(xy))). And so:
\begin{equation} P(E_1\ or\ \dots\ or\ E_{n}) = \sum_{r=1}^{n} (-1)^{r+1} Y_{r} \end{equation}
whereby, Y_{j} is the sum of P(x_n, … x_{j}) for combinations of j events. Try not to do this.