4.25 Distribution of blood types. All human blood can be “ABO-typed” as one of O, A, B, or AB, but the distribution of the types varies a bit among groups of people. Here is the distribution of blood types for a randomly chosen person in the United States:7
| Blood type | A | B | AB | O |
| U.S. probability | 0.42 | 0.11 | ? | 0.44 |
(a) What is the probability of type AB blood in the United States?
(b) Maria has type B blood. She can safely receive blood transfusions from people with blood types O and B. What is the probability that a randomly chosen person from the United States can donate blood to Maria?