If there are two outcomes of an event, the probability of one or the other happening is the sum of the separate probabilities.
P (a or b) = P (a) + P(b)

Example: A couple wants two children. If for each birth the probability of having a boy is ½, what is the probability they will have at least one boy? There are four likely outcomes:

BG BB GB GG
The probability of having one boy is
2/4 or 50%
The probability of having two boys (BB) is
1/4 or 25%
The probability of having at least one boy is
50% + 25% or 75%

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