Hi i need help with these homework questions if you could please help.

8) A random sample of 51 credit sales in a department store showed an average sale of $68.00. From past data, it is known that the standard deviation of the population is $27.00. For a 90% confidence interval, what is the size of the margin of error?

9) Find the 90% confidence interval using the information in Q8. Round your answers to at least two decimal places.

10) Now suppose that the standard deviation of $27 is not the population standard deviation, but rather has been calculated from the sample of 51 credit sales. Calculate the 90% confidence interval under this new assumption. Round your answers to at least two decimal places.