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Math Help - Confidence Interval and Standard Deviation

  1. #1
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    Confidence Interval and Standard Deviation

    I can't get these ones at all x.x please help


    ABC College Bookstore sells Ti83 calculators. The random number of calculators sold per day along with the probabilities is:

    # of calculators: 1, 2, 3, 4, 5
    Probability: 1/8, 1/6, 1/2, 1/12, 1/8

    What is the standard deviation for the number of calculators sold?


    ---

    Find a 95% confidence interval for the mean weight of bass caught in El Capitan lake if a sample of 300 fish had a mean weight of 77 ozs. with a standard deviation of 12 ozs.



    I've been trying to figure these out for a while and still can't get it right.
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  2. #2
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    Quote Originally Posted by Phil32686 View Post
    I can't get these ones at all x.x please help


    ABC College Bookstore sells Ti83 calculators. The random number of calculators sold per day along with the probabilities is:

    # of calculators: 1, 2, 3, 4, 5
    Probability: 1/8, 1/6, 1/2, 1/12, 1/8

    What is the standard deviation for the number of calculators sold?


    ---

    Find a 95% confidence interval for the mean weight of bass caught in El Capitan lake if a sample of 300 fish had a mean weight of 77 ozs. with a standard deviation of 12 ozs.



    I've been trying to figure these out for a while and still can't get it right.
    Q1 Use Var(X) = E(X^2) - [E(X)]^2. If you need more help, please show your working and say where you get stuck.

    Q2 Read this thread (specifically, reply #2): http://www.mathhelpforum.com/math-he...-interval.html

    You will find many questions on confidence intervals (with helpful replies) at MHF by using the Search tool (top right).
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  3. #3
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    Quote Originally Posted by mr fantastic View Post
    Q1 Use Var(X) = E(X^2) - [E(X)]^2. If you need more help, please show your working and say where you get stuck.

    Q2 Read this thread (specifically, reply #2): http://www.mathhelpforum.com/math-he...-interval.html

    You will find many questions on confidence intervals (with helpful replies) at MHF by using the Search tool (top right).

    For the first one, would I plug in probability for E and # of calculators for X?

    = (1/8)(1^2) - [(1/8*1)^2] + etc...

    And do that for all of the # of calculators and probabilities for each?

    -----

    For the Confidence Interval question, I got

    75.6421 < u < 78.3579
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  4. #4
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    Quote Originally Posted by Phil32686 View Post
    For the first one, would I plug in probability for E and # of calculators for X?

    = (1/8)(1^2) - [(1/8*1)^2] + etc...

    And do that for all of the # of calculators and probabilities for each?

    [snip]
    You calculate these things using the usual definitions:

    E(X) = (1)(1/8) + (2)(1/6) + ....

    E(X^2) = (1^2) (1/8) + (2^2) (1/6) + ....
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