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Math Help - z-scores?

  1. #1
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    Question z-scores?

    hi there,

    could anyone help me out? i really don't understand z-scores... i have a final coming up and i can't contact my teachers because they are all on holidays.
    for a given question, do i use the formula: z=(x-mean)/(standard deviation)? because when i am given the values for x, the mean, and standard deviation to find z, i always end up with a negative number or something that doesn't seem right. For example,

    x= 20
    Mean = 45
    Standard Deviation = 12

    How do I find the z-score using these values? It can't be just plugging the values into the formula. How do I use the chart? As well, what if I am not given an x values? What do I do?

    thanks.
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  2. #2
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    It is true that you simply plug these values into the formula above.

     z = \frac{X-\mu}{\sigma} = \frac{20-45}{12} = -2.08

    Now do you need to only find the z-score? Sometimes we need to find the probability of z being greater or less than this score.
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  3. #3
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    yes. how do i find the probability, whether it is greater than or less than the z-score?
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  4. #4
    Master Of Puppets
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    You need to use a table/chart, this online calculator is handy.

    Z table - Normal Distribution
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