The monthly closing stock price for a large technology firm for the first six months of the year are reported in the following table. 

Month                                                                          Price

January..........................................................................71

February........................................................................73

March.............................................................................76

April.................................................................................78

May..................................................................................81

June .................................................................................75

a. Calculate the sample mean and the sample standard deviation. 

b. Calculate the 90% confidence interval for the mean stock price of the firm, assuming that the stock price is normally distributed. 

c. What happens to the margin of error if a higher confidence level is used for the interval estimate?