A sample of 17 observations is selected from a normal population. The sample standard deviation is 21.75, and the sample mean is 36. a. Determine the standard error of the mean. (Round the final answer to 4 decimal places.) The standard error of the mean is b. Determine the 80% confidence interval for the population mean. (Round the final answers to 2 decimal places.) The 80% confidence interval for the population mean is between and c. If you wanted a wider interval, would you increase or decrease the confidence level? (Click to select)