Use mean and standard deviation to fit normal distributions and estimate population percentages with technology.
Find a normal cutoff to one decimal
Problem
Find the \(90\text{th}\text{-}\text{percentile}\) cutoff of \(N(100,~15)\). Give the \(\operatorname{invNorm}\) setup, approximate value with units, and cumulative interpretation.
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What this problem is really about
Inverse normal starts with cumulative area to the left of the desired cutoff. Convert percentile or top-tail language into that lower-tail probability, then enter area, mean, and positive standard deviation in order. Retain the computed cutoff until the final rounding, preserve the original units, and verify that the stated fraction lies at or below it.
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