A small Starbucks in middle-America serves, on average, 60 customers a night. A typical night in this town is long, about 10 hours. There's not much else to do. The manager observed customers in the store all night and she is wondering if she is allowing them to stay too long, just using the free internet. She has gathered data on how many customers are in the store at any one time, and believes this average to be 18 customers. These customers are eating food they purchased and drinking coffee, but she has calculated that each customer could finish eating and drinking in 30 minutes.How much extra time (more than they would need to eat and drink) is the average customer spending in the store?