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Math Help - Transform of norm dist. into Z question

  1. #1
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    Question Transform of norm dist. into Z question

    Random variable X ~ N(20,12)

    Find the values of a and b:

    P(X<a) = 0.40
    P(X>b) = 0.6915

    I can't get the answer in the text book and no worked examples are available. Any ideas would be greatly appreciated ...

    My workings are:

    P(X<a) = P(Z<((a-20)/(sqrt12)) using mu and sigma

    P(Z<((a-20)/(sqrt12)) = 0.40

    but dont have 0.4 in my Z tables.

    tre same for the other equation.
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  2. #2
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    help for the exercise

    Hello dojo,
    You can find my point of view about the exercise in annexe!
    Bye!
    Attached Thumbnails Attached Thumbnails Transform of norm dist. into Z question-normal-d.jpg  
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  3. #3
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    Thanks for your response. The answers you give are not what the answer book i have give. Their answers are 19.1 & 18.3 respectively
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  4. #4
    MHF Contributor matheagle's Avatar
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    One of you think 12 is the st deviation while the other thinks it's the variance.
    It could be either depending on what the authors use of that notation.
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