Example 9. The book has 300 pages. What is the probability that a randomly opened page will have an ordinal number that is a multiple of 5?
Solution. From the condition of the problem, it follows that the total number of equally possible elementary outcomes, forming a complete group of events, is $n=300$. Of these, $m=60$ are favorable for the occurrence of the specified event. Indeed, a number divisible by 5 has the form $5k$, where $k$ is a natural number, and $0<5k \leq 300$, hence $k \leq 300 / 5 \approx 60$. Therefore,

$$
P(A)=\frac{60}{300}=\frac{1}{5}=0.2
$$

where $A$ is the event "the page has an ordinal number divisible by 5".