Owner of the Bard restaurant has been running his shop for past few years and the quantity of food to be prepared each evening, according to him should be for 81 customers served each night with a standard deviation of 9. Seeing fall in sales for past weeks he thought of reaffirming his belief .A sample of 36 nights was taken for customers served and the mean was found to be 71. Do you think shop owner has enough evidence to believe that sales have declined recently?
State and test the relevant hypothesis at 90% level of confidence.