, 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)
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.