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Math Help - The problem involving normal distribution

  1. #1
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    The problem involving normal distribution

    The topic is Normal(Gaussian) distribution
    I need to find the value of x for such expression:

    mean is 5. standard deviation is 4

    P(x<X<9)=0.2

    I tried to evaluate the result by Z factor using:

    phi((x-5)/4)<X<phi((9-5)/4)=0.2

    But obtained different result.

    the answer must be 6.44
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  2. #2
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    Re: The problem involving normal distribution

    What "result" are you trying to get? In other words, what question are you asking? To find "x" such that P(x< X< 9)= .2? Yes, when x= 9, z= (9- 5)/4= 1. What does your Normal Table give for the probability that z< 1? What must be subtracted from that to get 0.2? What z value corresponds to that normal probability? And what x value is that?

    You say you "obtained different result". What result did you obtain?
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  3. #3
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    Re: The problem involving normal distribution

    The probability of z<1 is 0.84134. phi(1)=1-phi(-1)=1-0.15866.

    0.84134-P(X<x)=0.2

    P(X<x)=0.64134 But such probibilty doesn't exist in Normal table. What is the mistake here?
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