Once we are able to develop a basic fundamental probability, we can begin to build on that. One way to do that is to look at situations that will have you adding probabilities or more precisely using the additional rule or approach to probabilities. This happens when we encounter questions that put probabilities together with an OR statement. Let me give you an example, Suppose we studied 100 (22 year olds) to find out about their debt situation. We studied 35 individuals who had college degrees of which 25 had no debt and we studied 65 individuals with no degree finding 12 with no debt. Now we would like to know the probability of selecting an individual from this group and finding that they have a college degree or they have no debt? Now because the question asked an OR statement . (College degree or no debt) This require that use the additional rules which states: P(A or B) = P(A) P(B) - P(A and B) The reason we want to subtract the Probability of A and B is because we can not double count probabilities. Remember a probability is always and forever going to be a number between 0