Troista Mobile Accessories sells mobile apps on their web site. If a customer spends on average $12 per visit and visits the Web site 20 times each year, what is the average nondiscounted gross profit during a customer's lifetime? Given that Troista makes a margin of 60 percent on the average bill, with 25 percent of customers not returning each year. 

• $75 
• $576
• $360 
• $30