Example 3. Two dice are rolled, and the sum of the points on the upper faces is calculated. Find the probability of event $B$, which consists of the sum of the points on the upper faces of the dice being 9.
Solution. In this trial, there are $6^{2}=36$ equally likely elementary outcomes (see Table 1.1). Event $B$ is favored by 4 outcomes: $(3 ; 6),(4 ; 5),(5 ; 4),(6 ; 3)$, therefore

$$
P(B)=\frac{4}{36}=\frac{1}{9}
$$