Practice Test Question, Probability with Percents, Please Help?

Question

Mrs. Jones surveys her class about their siblings. In the class, 75% of the students have a brother, 82% have a sister, and 65% have both a brother and a sister. What is the probability that a student has a brother or a sister?

Answer 1

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.