You use multiplication when the two events are independent of one another. In the case of flipping coins, the probability that a coin is heads-up is 1/2. So the probability of flipping two coins heads-up is (1/2)(1/2) = 1/4. You add distinct probabilities when you want to find the probability of an event that encompasses several probabilities, such as: What is the probability of drawing an 8 or a 9 from a standard deck of 52 cards? It is the same as the probability of drawing an 8 plus the probability of drawing a 9.