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19 November, 14:03

Assuming a normal distribution with a true mean of 5 Inches and a standard deviation of 2.2 Inches, what is the probability (in percentage) that future measurements will fall above 4.35 Inches?

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  1. 19 November, 15:49
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    P = 61.41 %

    Explanation:

    Let's call X to the random variable.

    X ~ N (μ,σ)

    Where μ is the mean and σ is the standar deviation.

    They are asking us about P (X>4.35 inches)

    This probability is equal to:

    P (X>4.35 inches) = 1 - P (X≤4.35 inches)

    Writing ≤ or < is the same because normal distribution is a continuos random variable.

    Let's find first P (X≤4.35 inches). We need to turn X into a N (0,1) standardizing the variable X

    We do this by subtracting the mean and dividing by the standard deviation

    P (X≤4.35 inches) = P [ (X-μ) / σ ≤ (4.35 inches - μ) / σ]

    (X-μ) / σ = Z ⇒Z ~ N (0,1)

    (4.35 inches - μ) / σ = (4.35 inches-5 inches) / 2.2 inches = - 0.295454

    P (X≤4.35 inches) = P (Z≤-0.295454)

    The probability for Z we can find it on a table

    P (Z≤-0.295454) = Ф (-0.295454) = 0.3859 = P (X≤4.35 inches)

    Then P (X>4.35 inches) = 1 - P (X≤4.35 inches) = 1 - 0.3859 = 0.6141 = 61.41 %
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