The conditional probability of event given another event occurs is $$ P(a\vert b) = \frac{p(a \land b)}{p(b)} \text{ if } p(b) > 0$$
The conditional probability of event given another event occurs is $$ P(a\vert b) = \frac{p(a \land b)}{p(b)} \text{ if } p(b) > 0$$