A sample of12 observations is selected from a normal population. The sample standard deviation is 20.50, and the sample mean is 21. 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 90% condence interval for the population mean. (Round the final answers to 2 decimal places.) The 90% confidence interval for the population mean is between |:| and |:|. c. If you wanted a wider interval, would you increase or decrease the condence level? (Click to select) Decrease Increase