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Math Help - Finding an unknown value in normal distribution.

  1. #1
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    Finding an unknown value in normal distribution.

    The continuous random variable Y is normally distributed with mean 15 and standard deviation 6
    (a) Find P(X21)

    (b) Find the value of d such that P(15<X<d)=0.3152

    How do you answer part (b)?
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  2. #2
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    Quote Originally Posted by CSG18 View Post
    The continuous random variable Y is normally distributed with mean 15 and standard deviation 6
    (a) Find P(X21)

    (b) Find the value of d such that P(15<X<d)=0.3152

    How do you answer part (b)?
    Note that the mean is 15. So you have:

    Pr(X < d) - 0.5 = 0.3152 => Pr(X < d) = 0.8152. This is now a simple inverse normal problem.
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    I do not understand where 0.5 comes from. Could you explain please? Also how would you reach the final answer..I guess I do not know how to solve a simple inverse normal problem..

    Thanks!
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    Quote Originally Posted by CSG18 View Post
    I do not understand where 0.5 comes from. Could you explain please?

    Thanks!
    Are you aware that Pr(X < mean) = 0.5 for a normal distribution?

    Quote Originally Posted by CSG18 View Post
    [snip from edited reply] Also how would you reach the final answer..I guess I do not know how to solve a simple inverse normal problem..

    Thanks!
    Your textbook and class notes should have examples of how to do that. Have you reviewed that material?
    Last edited by mr fantastic; March 12th 2010 at 02:43 PM. Reason: Merged posts
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    Well our teacher has not taught us normal distribution, so I have to learn it by myself. It's ok I'll figure it out.

    Thanks
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    Quote Originally Posted by CSG18 View Post
    Well our teacher has not taught us normal distribution, so I have to learn it by myself. It's ok I'll figure it out.

    Thanks
    Fnd the value z* of Z such that Pr(Z < z*) = 0.8152. Then z* = (d - 15)/6. Solve for d.

    By the way, why are you attempting questions that your teacher has not taught you how to do?
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    It's going to be on my exam, but I don't think my teacher will have time to teach us it. So I will have to learn it.

    Thanks I get it now!
    Last edited by CSG18; March 12th 2010 at 03:37 PM.
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    Quote Originally Posted by CSG18 View Post
    It's going to be on my exam, but I don't think my teacher will have time to teach us it. So I will have to learn it.

    The only thing I don't understand is how to get z*. How does 0.8152 get to 0.897?
    How can an exam possbly test you on material that has not been taught? Unfortunately, there are many things the question you posted expects you to already understand. MHF cannot act as a surrogate teacher. If you have resolved to self-learn the normal distribution I suggest you obtain a decent textbook that covers the material and start from the very beginning.

    I also suggest you discuss the issues raised in this thread with your teacher.
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  9. #9
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    I understand, thanks for your help. I'm doing the IB and in my first year my original maths teacher was supposed to teach my class normal distribution but didn't, so now my new teacher probably won't have time to teach it.
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  10. #10
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    Quote Originally Posted by CSG18 View Post
    I understand, thanks for your help. I'm doing the IB and in my first year my original maths teacher was supposed to teach my class normal distribution but didn't, so now my new teacher probably won't have time to teach it.
    Speak to your new teacher about this.
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