In the International Journal of Retail and Distribution Management (Vol. 39, 2011) survey of Black Friday shoppers, one question asked was, "How many hours do you usually spend shopping on Black Friday?" Suppose that the population standard deviation is known to be 2.47 and a 90% confidence interval for the mean hours spent shopping on Black Friday is to be constructed.

How many Black Friday shoppers should be included in a sample designed to estimate the average number of hours spent shopping on Black Friday if you want the estimate to deviate no more than .4 hour from the true mean?