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Mrs. Jones surveys her class of 20 students about their siblings. In the class, 15 have a brother, 16 have a sister, and 12 have both a brother and a sister. What is the probability that a student has
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Mrs. Jones surveys her class of 20 students about their siblings. In the class, 15 have a brother, 16 have a sister, and 12 have both a brother and a sister. What is the probability that a student has a brother OR a sister?

User Adrian Baddeley
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about 10 % of people might have a brother or sister 
User Gaurav Aggarwal
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P(Brother or Sister) = .75 + .82 - .65 P(Brother or Sister) = .92 = 92% The reason you subtract the .65 is that scenario does not fit the rule. The rule is the probability of having a brother OR a sister. The .65 represents the kids who have both not either.
User DrSammyD
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