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For a confidence level of 95% the sample means fall within 1.96 standard errors of the population mean.
This is used to calculate the confidence interval, where mean is 90.6, the population standard deviation is `sigma = 8.9` and the sample size is n = 92.
The confidence interval in which `mu` lies is given by `bar X -1.96*(sigma/sqrt n)` and `bar X + 1.96*(sigma/sqrt n)`
`90.6 - 1.96*(8.9/sqrt 92) = 88.78`
`90.6 + 1.96*(8.9/sqrt 92) = 92.42`
The confidence interval is `88.78 < mu < 92.42`
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