Example 6.1. A ball is randomly drawn from an urn containing 4 white, 9 black, and 7 red balls. What is the probability of drawing a white ball?
Solution. Here, the elementary outcome is the extraction of any ball from the urn. The number of all such outcomes is equal to the number of balls in the urn, i.e., \( n=20 \). The number of outcomes favorable to the appearance of a white ball (event \( A \)), obviously, is equal to the number of white balls in the urn, i.e., \( m=4 \). Therefore, by formula (1) we find:

\[
P(A)=\frac{4}{20}=\frac{1}{5}
\]