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Daniel Fellows
Lv 5
Daniel Fellows asked in Science & MathematicsMathematics · 8 years ago

How do I find the standard deviation in normal distribution?

Assuming an average score on the ACTs is 21, what is the standard deviation if 5% of students who take the test score a 34?

Update:

@Guillermo

I followed you up until I reached the "σx = 7/1.644853" part. How did you get the 7?

1 Answer

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  • Guillermo
    Lv 7
    8 years ago
    Favorite Answer

    Let X the scores of a test.

    X ∼ n(21; σx)

    P(X ≥ 34) = 0.05

    P((X - 21)/σx ≥ (34 - 21)/σx) = 0.05

    P(Z ≥ z) = 0.05

    z = 1.644853

    1.644853 = (34 - 21)/σx

    σx = 7/1.644853

    σx = 4.255699445, answer!

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