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The average age of a vehicle registered in the U.S. is 8 years, or 96 months. If a random sample of 36 vehicles is selected, find the probability that the mean of their age is between 98 and 100 months. Assume the standard deviation for the population is 15. (sample mean)

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    calculate the z scores:
    z= (x-xbar)/(σ/sqrt(n))
    where xbar=sample mean, σ=standard deviation of population, n=sample size, x=raw score (i.e. 98, 100)

    now refer to the z chart:
    http://www.thebeststatistics.info/sta/table_zvals.html

    since you want the probability that the mean is between 98 and 100 months, I'd use the p-values on the right column (i.e. z-score to infinity) and take the difference of the p-values to get the area between 98 and 100.

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