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Math Help - binomial random variable

  1. #1
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    binomial random variable

    Each of 1000 people in a place independently has a certain
    disease with probability 0.005.

    i Find the probability that at least one person has the disease
    ii. Find the probability that there is more than one person has the disease, if we know that there is at least one person having the disease
    iii. If one of the 1000 people knows that he has the disease. What does
    the probability that more than one person (including himself) has the
    disease

    Please help me by not just telling the answers but explaining why as I don't understand anything.

    For example first question is it 1-(1-0.005)=0.995??????
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  2. #2
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    Quote Originally Posted by someone21 View Post
    Each of 1000 people in a place independently has a certain
    disease with probability 0.005.

    i Find the probability that at least one person has the disease

    Mr F says: 1 - Pr(X = 0) = ....

    ii. Find the probability that there is more than one person has the disease, if we know that there is at least one person having the disease

    Mr F says: Use the conditional probablity formula on {\color{red}Pr(X > 1 | X \geq 1)}.

    iii. If one of the 1000 people knows that he has the disease. What does
    the probability that more than one person (including himself) has the
    disease

    Please help me by not just telling the answers but explaining why as I don't understand anything.

    For example first question is it 1-(1-0.005)=0.995??????
    Let X be the the random variable number of people who have the disease.
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  3. #3
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    What about iii?


    And do we use binomial probability here??
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  4. #4
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    Quote Originally Posted by someone21 View Post
    What about iii?


    And do we use binomial probability here??
    I can't see the difference between ii and iii ......
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  5. #5
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    Please really help me

    how to you still solve (ii) or(iii)

    I don't understand how to do calculation
    Last edited by someone21; October 8th 2008 at 09:21 AM.
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  6. #6
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    Quote Originally Posted by someone21 View Post
    Please really help me

    how to you still solve (ii) or(iii)

    I don't understand how to do calculation
    X ~ Binomial (n = 1000, p = 0.005.

    Calculate Pr(X > 1 | X \geq 1) using the usual conditional probability formula.
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