Probability

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?

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Keep in mind, the question is asking what probability has a brother OR a sister.  The 65%, that have both, does not fit the situation.

P(brother or sister) = 0.75 + 0.82 - 0.65   {75% have brother, 82% have sister, 65% have both which doesn't fit}
= 0.92   {added and subtracted}

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