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Sum Rule for Probability

A method for finding the probability that either or both of two events occurs.

 Addition Rule: If events A and B are mutually exclusive (disjoint), then P(A or B) = P(A) + P(B) Otherwise, P(A or B) = P(A) + P(B) – P(A and B) Example 1: mutually exclusive In a group of 101 students 30 are freshmen and 41 are sophomores. Find the probability that a student picked from this group at random is either a freshman or sophomore. Note that P(freshman) = 30/101 and P(sophomore) = 41/101. Thus P(freshman or sophomore) = 30/101 + 41/101 = 71/101 This makes sense since 71 of the 101 students are freshmen or sophomores. Example 2: not mutually exclusive In a group of 101 students 40 are juniors, 50 are female, and 22 are female juniors. Find the probability that a student picked from this group at random is either a junior or female. Note that P(junior) = 40/101 and P(female) = 50/101, and P(junior and female) = 22/101. Thus P(junior or female) = 40/101 + 50/101 – 22/101 = 68/101 This makes sense since 68 of the 101 students are juniors or female. Not sure why? When we add 40 juniors to 50 females and get a total of 90, we have overcounted. The 22 female juniors were counted twice; 90 minus 22 makes 68 students who are juniors or female.