All human blood can be typed as either O, A, B, or AB. The distribution of the types varies a bit with race. Choose an African American person at random. Here are the approximate probabilities that the person you choose will have blood type O, B, or AB:
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$$\begin{array} { | l | l | l | l | l | }
\hline \text { Blood type } & \text { O } & \text { A } & \text { B } & \text { AB } \
\hline \text { Probability } & 0.50 & ? & 0.20 & 0.05 \
\hline
\end{array}$$ What is the probability that the person chosen has blood type A?

Question

All human blood can be typed as either O, A, B, or AB. The distribution of the types varies a bit with race. Choose an African American person at random. Here are the approximate probabilities that the person you choose will have blood type O, B, or AB:
?
?
$$\begin{array} { | l | l | l | l | l | } 
\hline \text { Blood type } & \text { O } & \text { A } & \text { B } & \text { AB } \
\hline \text { Probability } & 0.50 & ? & 0.20 & 0.05 \
\hline
\end{array}$$ What is the probability that the person chosen has blood type A?

Quizplus · Accepted Answer

Since the probabilities of O, B, and AB blood types are given, we can use the fact that the sum of all probabilities must equal 1 to find the probability of type A. The probability of A blood type is: 
$P(	ext{A}) = 1 - P(	ext{O}) - P(	ext{B}) - P(	ext{AB}) = 1 - 0.50 - 0.20 - 0.05 = 0.25$
Therefore, the best choice is B.

All Human Blood Can Be Typed as Either O, A