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Math Help - quick question on probability of AT LEAST and EXACTLY

  1. #1
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    quick question on probability of AT LEAST and EXACTLY

    The probability that student A will fail is .5, the probability that student B will fail is .2, and the probability that both will fail is .1. So..

    P(A)=.5 P(B)=.2 P(AB)=.1

    What is the probability that at least one student will fail?
    Exactly one student will fail?

    I did .5+.2-.1= .6 for P of at least one. I'm not sure if this is right or if it is for exactly one student instead. Any help please?
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  2. #2
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    Quote Originally Posted by holly123 View Post
    The probability that student A will fail is .5, the probability that student B will fail is .2, and the probability that both will fail is .1. So..

    P(A)=.5 P(B)=.2 P(AB)=.1

    What is the probability that at least one student will fail?
    Exactly one student will fail?

    I did .5+.2-.1= .6 for P of at least one. I'm not sure if this is right or if it is for exactly one student instead. Any help please?
    You require \Pr(A  \cup B). There is a formula for this, which you have clearly used. Your calculation is correct.
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    Thanks, I was just wondering if I should do that for "at least one" or "exactly one" or both?
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    Quote Originally Posted by holly123 View Post
    Thanks, I was just wondering if I should do that for "at least one" or "exactly one" or both?
    Exactly one means calculating Pr(A and B') + Pr(A' and B).
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