Multiple random variables? No problem. Learn how to calculate probabilities using a joint distribution and how to calculate a marginal (univariate) pmf from a joint pmf.
Errata: if you sum over the entire pdf, you should get 1. (I mistakenly wrote 0.) This is just like in the univariate case: sum over the entire pdf = 1.
![](https://s2.save4k.ru/pic/Hxs4GSPActc/maxresdefault.jpg)