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Math Help - Simple random sample

  1. #1
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    Simple random sample

    Hi I just need a quick help on the question below which I ran into some difficulty while attempting.

    A simple random sample of size n=18 is taken from a population with mean 16 and variance 5. the population distribution is assumed to be Normal. Let X be the mean of the sample. Answer the following questions.

    (a) What is the distribution of X?
    (b) What is P( X < 17 )?
    (c) Suppose you need to choose a sample of size n such that
    P( |X-16| > 1.5 ) < 0.05. What is the minimum n required to achieve this?

    For (a) X = mean of sample. I took X~N ( 16, 5/18) = N (16, 0.278)

    As for (b) I took P(X < 17) = P[ Z < (17-18)/Square root of 0.278 ]
    = P ( Z < - 1.8966 )

    Now, for part (b), I am not really sure how to continue. Should I minus the answer with 1? I am quite confused with the sign '<' and '>'

    And for part (c), I am not really sure how to continue with an absolute value. Please help!
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  2. #2
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    Joined
    Aug 2009
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    Actually I made a mistake
    In part (b) I should have taken P(X< (17-16)/Square root of 0.278)
    Giving me P(X<1.9)
    So is the answer 0.4713.

    I am quite confused about part (c) as well
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