Suppose a sample space “S” consists of “N” outcomes and the probability of each outcome is equally likely. The two events exist from the same sample space containing n_{1 }and n_{2 }outcomes respectively. Events are named as “x” and “y”. Both the events have nothing in common so called these events as mutually exclusive events. As both the events are mutually exclusive events therefore, their union contains (n_{1} + n_{2}) outcomes. Probability of x & y can be write as by using the classical formula of probability,

[P(Xcup Y) = frac{Xcup Y}{S (sample space))}]

[P(X cup Y) = frac{n_{1}+n_{2}}{N} = frac{n_{1}}{N}+frac{n_{2}}{N}= P(X)+ P(Y)]

Similarly, if there are “n” number of mutually exclusive events then,

[P(X_{1} cup X_{2}cup…..X_{n} ) = P(X_{1})+ P(X_{2})+ P(X_{3})……P(X_{n}) = sum_{i=1}^{n}(X_{i})]