Results 1 to 2 of 2

Math Help - Mean Help!

  1. #1
    Member looi76's Avatar
    Joined
    Jan 2008
    Posts
    185

    Mean Help!

    Question:
    The pirce of a CD is denoted by $x. For 60CDs bought in different stores it is found that \sum(x-12) = 53.60. Calculate the mean price of these CDs. The mean price of a further 40 CDs is found to be $11.64. The mean price of the 100 CDs.


    Attempt:
    \sum^{60}_{i=1}(x-12) = 53.40

    60\times12+53.40 = 773.40

    \sum\bar{x} = 773.40

     \bar{x} = \frac{773.40}{60} = \$12.89

    Mean of 100 CDs =  \frac{12.89 + 11.64}{2} = \$12.27

    The mean for 60 CDs is correct! but for 100 CDs is wrong. Where did I do wrong?
    Follow Math Help Forum on Facebook and Google+

  2. #2
    GAMMA Mathematics
    colby2152's Avatar
    Joined
    Nov 2007
    From
    Alexandria, VA
    Posts
    1,172
    Awards
    1
    Quote Originally Posted by looi76 View Post

    Mean of 100 CDs =  \frac{12.89 + 11.64}{2} = \$12.27

    The mean for 60 CDs is correct! but for 100 CDs is wrong. Where did I do wrong?
    You need to take a weighted average of the two means like so:

    \bar{x}_{100} = \frac{60*\bar{x}_{60}+40*\bar{x}_{40}}{100}
    Follow Math Help Forum on Facebook and Google+


/mathhelpforum @mathhelpforum