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Math Help - Standard deviation calculation

  1. #1
    Newbie
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    Standard deviation calculation

    Number of cars sold daily - Probability
    0 - 0.1
    1 - 0.11
    2 - 0.2
    3 - 0.23
    4 - 0.3
    5 - 0.06


    a) find the mean, variance and standard deviation of the number of cars sold in total during 2 days, assuming sales on the 2 days are independent of each other.


    Mean: used sum of (# of cars * probability) got 2.7 then just multiplied it by 2 and got 5.4 (right answer)


    variance: used sum (x-u)^2*probabilty thn mulitpled by 2 and got 3.98 (right answer)



    SD = sqrt 3.98 = 1.99**
    **in the answers, it says it should be 1.1877178? How can my variance be right but my standard deviation wrong when all you have to do is square root it? Is there a typo in the answer key or is there another formula to use?
    Last edited by hello_math_; December 12th 2009 at 01:12 AM.
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  2. #2
    Grand Panjandrum
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    Quote Originally Posted by hello_math_ View Post
    Number of cars sold daily - Probability
    0 - 0.1
    1 - 0.11
    2 - 0.2
    3 - 0.23
    4 - 0.3
    5 - 0.06


    a) find the mean, variance and standard deviation of the number of cars sold in total during 2 days, assuming sales on the 2 days are independent of each other.


    Mean: used sum of (# of cars * probability) got 2.7 then just multiplied it by 2 and got 5.4 (right answer)


    variance: used sum (x-u)^2*probabilty thn mulitpled by 2 and got 3.98 (right answer)



    SD = sqrt 3.98 = 1.99**
    **in the answers, it says it should be 1.1877178? How can my variance be right but my standard deviation wrong when all you have to do is square root it? Is there a typo in the answer key or is there another formula to use?
    either the book is wrong or you have reported the question incorrectly

    CB
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