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Universal Blood Donors. People with type O-negative blood are referred to as universal donors, although if you give type O-negative blood to any patient, you run the risk of a transfusion reaction due to certain antibodies present in the blood. However, any patient can receive a transfusion of O-negative red blood cells. Only 7.2\% of the American population have O-negative blood. If 10 people appear at random to give blood, what is the probability that at least one of them is a universal donor?

A Probability Teaser. Let's assume it is safe to say that people are either male or female at birth 18 and that each child born is equally likely to be a boy or a girl and that the sexes of successive children are independent. If we let BG mean that the older child is a boy and the younger child is a girl, then each of the combinations BB, BG, GB, and GG has probability \(0.25\). Ashley and Brianna each have two children. a. You know that at least one of Ashley's children is a boy. What is the conditional probability that she has two boys? b. You know that Brianna's older child is a boy. What is the conditional probability that she has two boys?