9. Xiao Ming spent 48 yuan to buy several exercise books at the retail price. If he bought them at the wholesale price, each book would be 2 yuan cheaper, and he could buy exactly 4 more books. Question: What is the retail price per book?

保留源文本的换行和格式，翻译结果如下：

9. Xiao Ming spent 48 yuan to buy several exercise books at the retail price. If he bought them at the wholesale price, each book would be 2 yuan cheaper, and he could buy exactly 4 more books. Question: What is the retail price per book?
9. Retail price per book 6 yuan
9.【Solution】See the figure below, with the horizontal line representing the number of books, and the vertical line representing the unit price. Since the area of the yellow part is equal to the area of the green part, the width of the yellow part is twice the height of the green part. Let the wholesale price be $\mathrm{x}$ yuan (the height of the green rectangle in the figure),
then we have: $\mathrm{x} \times(2 \mathrm{x}+4)=48$, which simplifies to $\mathrm{x} \times(\mathrm{x}+2)=24=4 \times 6=4 \times(4+2)$, so, $x=4$ (yuan), and the retail price is $x+2=6$ (yuan)