Suppose that the number of customers entering a department store in a day is a random variable with mean of 21 customers/day. suppose that the amount of money spent by each one of these customers is a random variable is governed by the following probability: with probability 1/5 a customer spends $20, with probability 2/5 a customer spends $10, with probability 2/5 a customer spends $5. the amounts spent by different customers are statistically independent. also assume that the amount spent by each customer is independent of the number of customers to enter the store. calculate the average cumulative amount spent by all customers that visit the store in a day.