Example 7. A letter is randomly selected from the word "differential". What is the probability that this letter will be: a) a vowel, b) a consonant, c) the letter $4?$
Solution. The word differential has 12 letters, of which 5 are vowels and 7 are consonants. The letter ч is not in this word. Let's denote the events: $A$ - "vowel letter", $B$ - "consonant letter", $C$ - "letter ч". The number of favorable elementary outcomes: $m_{1}=5$ - for event $A, m_{2}=7$ - for event $B$, $m_{3}=0$ - for event $C$. Since $n=12$, then

$$
P(A)=\frac{5}{12} \approx 0.417 ; \quad P(B)=\frac{7}{12} \approx 0.583 ; \quad P(C)=0 .
$$