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A sample of 20 observations is selected from a normal population for which the population standard deviation is known to be 6. The sample mean is 25. a. Determine the standard error of the mean. (Round the final answer to 3 decimal places.) The standard error of the mean is |:| b. Explain why we can use formula (81) to determine the 90% condence interval, even though the sample size is less than 30. (Click to select) v c. Determine the 90% condence interval for the population mean. (Round the intermediate calculation to 3 decimal places. Round the final answers to 3 decimal places.) The 90% confidence interval for the population mean is between |:| and |:|