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Question from Jim, a student:

Blood Type Male Female Total
O 80 370 450
A 150 250 400
B 50 50 100
AB 20 30 50
Total 400 600 1000

What is the probability that the person selected is female if the person's blood type is O?
What is the probability that the person selected has blood type AB if the person is male?
What is the probability that the person selected is a female with blood type O?

Hi Jim,

Let me try two different questions.

What is the probability that the person selected has blood type O if the person is female?

Since you know the person is a female you can concentrate on the female column. The total number of females is 600 and the number of females with type O blood is 370 so the probability that the person selected has blood type O if the person is female is 370/600.

What is the probability that the person selected is a male with blood type A?

Here you don't know if the person is male or female and you don't know the blood type, you are just selecting someone at random form the 1000 people. There are 150 males with type A blood so the probability that the person selected is a male with blood type A is 150/1000.

Penny

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