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Constructing a Confidence Interval In Exercises 17-20, you are given the sample mean and the sample standard deviation. Assume the population is normally distributed and use the r-distribution to find the margin of error and construct a 95% confidence interval for the population mean. Interpret the results.20. Mobile Device Repair Costs In a random sample of 12 mobile devices, the mean repair cost was $90.42 and the standard deviation was $33.61