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Math Help - calculating sum

  1. #1
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    calculating sum

    As part of a statistics project, Gill collected data relating to the length of time, to the
    nearest minute, spent by shoppers in a supermarket and the amount of money they spent.
    Her data for a random sample of 10 shoppers are summarised in the table below, where t
    represents time and m the amount spent over 20.
    Calculate Stt, Smm and Stm.
    (You may use Σt2 = 5478 Σm2 = 2101 Σtm = 2485)

    heres the table, I know how to calculate the above sums, but what I cant understand is how to get the sum of m ?

    from the table would I not just add all the values under m? I get 216, but this is incorrect?

    the correct sum for m = 61 and I have no idea how to get that,;;

     \sum m = 17+ 17+1+4+12+11+27+6+20+6 = 219 why is this not correct?

    Also I have taken into account the minus signs since m stands for the amount over 20, than -3 is 20-3 = 17

    right?
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  2. #2
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    Quote Originally Posted by Tweety View Post
    Also I have taken into account the minus signs since m stands for the amount over 20, than -3 is 20-3 = 17
    right?

    If you work like this then maybe ,as -3 is 20-3, 17 is 20+17=37 ??? If i understand well the last line of your exercise because english isn't my mother language

    Are you sure the correct answer is 61?
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  3. #3
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    yes certain that the right answer is 61,
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  4. #4
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    Heres the answer book;
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  5. #5
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    Quote Originally Posted by Tweety View Post
    Calculate Stt, Smm and Stm.
    (You may use Σt2 = 5478 Σm2 = 2101 Σtm = 2485)

    heres the table, I know how to calculate the above sums, but what I cant understand is how to get the sum of m ?

    from the table would I not just add all the values under m? I get 216, but this is incorrect?

    the correct sum for m = 61 and I have no idea how to get that,;;

     \sum m = 17+ 17+1+4+12+11+27+6+20+6 = 219 why is this not correct?
    Where did you get those numbers? In the table you show m= 17 only occurs once,not twice. There is no "1" or "4" in the table, etc.

    Also I have taken into account the minus signs since m stands for the amount over 20, than -3 is 20-3 = 17

    right?
    Last edited by mr fantastic; May 19th 2009 at 07:34 AM. Reason: Replaced [math] with [quote] to fox the bottom quote.
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  6. #6
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    Quote Originally Posted by HallsofIvy View Post
    Where did you get those numbers? In the table you show m= 17 only occurs once,not twice. There is no "1" or "4" in the table, etc.
    since m stand for the amount spent over 20, I thought the minus sign means you have to subtract the amount from 20, but apparently not,

    you just add up the numbers including the minus signs which = 61

    thanks.
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