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In probability, the addition law states that the possibility of events A or B happening is given by the probability of event A added to the probability of event B, minus the probability of them both happening. (If the events are mutually exclusive, then the probability of them both happening will be 0. So it still works). This can be written as -

P(A or B) = P(A) + P(B) - P(A U B)

Example

I roll a die. The probability of me rolling a 1 is 1/6, the probability of me rolling a 5 is 1/6. The probabilty of me rolling a 1 and a 5 is zero. Therefore -

P(1 or 5) = 1/6 + 1/6 - 0
P(1 or 5) = 2/6.

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