Bookstore Promotion - Paid Books Calculation

A bookstore has a special promotion where every 3rd book you purchase is free. If a customer buys a total of B books, including free ones, and pays for every book except for the free ones, how many books did the customer actually pay for?

Write an equation to determine the number of books paid for based on the total number of books purchased. If the customer bought 15 books in total, how many did they pay for?