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SD Problem - GMATPrep

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queenisabella Rising GMAT Star Default Avatar
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SD Problem - GMATPrep Post Sat Mar 31, 2012 6:34 pm
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  • Lap #[LAPCOUNT] ([LAPTIME])
    70, 75, 80, 85, 90, 105, 105, 130, 130, 130
    The list shown consists of the times, in seconds, that it took each of the 10 schoolchildren to run a distance of 400 meters. If the standard deviation of the 10 running times is 22.4 seconds, rounded to the nearest tenth of a second, how many of the 10 running times are more than 1 standard deviation below the mean of the 10 running teams?
    A) 1
    B) 2
    C) 3
    D) 4
    E) 5

    OA is B
    Source: GMATPrep

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    Bill@VeritasPrep GMAT Instructor
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    Post Sat Mar 31, 2012 6:44 pm
    The standard deviation tells us distance from the mean, so we need to find the mean. The sum of the 10 times is 1000, so the mean is 100.

    1 standard deviation below the mean is 100 - 22.4 = 77.6

    Only 70 and 75 are below 77.6

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