If we have two events, A and B, that we are interested in, we often want to know the probability that either A or B occurs. Note the use of the word "or," the key to this rule. The "or" is what we call an "inclusive or." In other words, either one event can occur or both events can occur.

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Subject 3. Addition Rule for Probabilities: the Probability that at Least One of Two Events Will Occur
If we have two events, A and B, that we are interested in, we often want to know the probability that either A or B occurs. Note the use of the word "or," the key to this rule. The "or" is what we call an "inclusive or." In other words, either one event can occur or both events can occur. Such probabilities are calculated using the addition rule for probabilities. P(A or B) = P(A) + P(B) - P(AB) The logic behind this f