2.2 Equiprobable Spaces

A finite probability space $S$ where each element of $S$ has the same probability is called an equiprobable space

In particular, if $S$ contains $n$ elements then the probability of each element of $S$ is $\frac{1}{n}$

For an event $A$ of an equiprobable space $S$ , $P (A) = \frac{number of elements in A}{number of elements in S} = \frac{| A |}{| S |}$

Example

Let a fair die be tossed
$A =$ The event that an odd number appears
Find $P (A)$

$A = {1, 3, 5}$
$P (A) = \frac{| A |}{| S |} = \frac{3}{6} = \frac{1}{2}$