suppose that the number of customers who enter a supermarket each hour is normally distributed with a mean of 520 and a standard deviation of 210. the supermarket is open 15 hours per day. what is the probability that the total number of customers who enter the supermarket in one day is greater than 9100? assume that number of customers who enter the supermarket each hour is independent. (hint: calculate the average hourly number of customers necessary to exceed 9100 in one 15-hour day.)