The last Harry Potter novel has just arrived and by 11 am a line has formed outside the crowded bookstore. Knowing you have class at noon, you ask the guard at the door how long you can expect to wait. She tells you that she is permitted to let 6 people into the store only after 6 people have left; customers are leaving at the rate of 2 customers per minute; and she has just let 6 new customers in. Also each customer spends an average of 15 minutes browsing at books and 10 minutes waiting in line to check out. How many minutes are there between groups of customers entering the book store?