9. Emma is earning reward points, \( r \), for every book, \( b \), she sells at a book fair. She is awarded 4 points for each book sold.

Emma is in the business of turning pages into points! For every book she sells at the book fair, she earns a sweet 4 reward points. So if she sells, let’s say, 10 books, she’ll walk away with a whopping \( r = 4 \times 10 = 40 \) points! It’s like adding a cherry on top of her book-selling sundae! To maximize her reward points, Emma should keep her book display inviting and well-organized, maybe even host a little reading corner for potential buyers. A common mistake would be not tracking how many books she sells during the fair. Keeping a tally can help her calculate her total points quickly and avoid any surprises at the end of the day!