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Math Help - mean

  1. #1
    Senior Member furor celtica's Avatar
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    mean

    the price of a CD is denoted by $x. for 60 CDs bought in different stores it is found that sigma(x-12) = 53.40. Calculate the mean price of these CDs. The mean price of a further 40 CDs is found to be $11.64. Find the mean price of the 100 CDs.

    The first problem was easy, divide 53.40 by 60 and add 12. BUt with the second one i don't know what method to use i know it must be more complicated than adding the two means and dividing by two
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  2. #2
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    if x is the first group of cds and y is the second group then

    the mean of the total group is
    \frac{\sum X + \sum Y}{n_x + n_y}

    You know that
    {\sum X} = 60 * \bar{X}
    {\sum Y} = 40 * \bar{Y}

    You say you calculated xbar already (although i have not seen and dont follow that method). you are given ybar.
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  3. #3
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    mr fantastic's Avatar
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    Quote Originally Posted by furor celtica View Post
    the price of a CD is denoted by $x. for 60 CDs bought in different stores it is found that sigma(x-12) = 53.40. Calculate the mean price of these CDs. The mean price of a further 40 CDs is found to be $11.64. Find the mean price of the 100 CDs.

    The first problem was easy, divide 53.40 by 60 and add 12. BUt with the second one i don't know what method to use i know it must be more complicated than adding the two means and dividing by two
    \overline{x} = \frac{60 \, \overline{x_1} + 40 \, \overline{X_2}}{60 + 40}.
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