Often, when solving probability problems, you do not need to identify each specific outcome of an experiment. Instead, you may want to consider the outcomes of an event only as happening or not happening . For example, rather than saying the outcome of each flip of a coin is heads or tails, you could say the outcomes are heads or not heads. Recall from Lesson 3A that, if the probability of an outcome happening is denoted P( A ), then, the probability of the outcome not happening is denoted P(A') . This is illustrated in the following probability tree diagram.
In Lesson 3A, you learned that P(A) + P(A') = 1 . This is accurate for the probability tree diagram also. The branches represent all the outcomes, so the probabilities shown on the branches represent all the probabilities for a total of 1.