In a sample of 18 days taken at Town Talk Bakery, it was found that an average of 56 cakes per day were sold. From past surveys, the population standard deviation was found to be 18 cakes per day.

a. Determine the standard error of the mean number of cakes sold per day. (Round the final answer to 3 decimal places.)

The standard error of the mean is

b. Determine the 80% confidence interval for the population mean number of cakes sold per day. (Round the z value to 2 decimal places. Round the final answers to 3 decimal places.)

The 80% confidence interval for the population mean is between ... and ...

c. Determine the 90% confidence interval for the population mean number of cakes sold per day. If you wanted a wider interval, would you increase or decrease the confidence level?