15. Class (8) of the third year of junior high school still has a class fund of $m$ (where $m$ is a positive integer less than 400) yuan, and plans to buy 1 photo album for each student. A wholesale and retail stationery store stipulates: if 50 or more photo albums are purchased, they can be sold at the wholesale price; if fewer than 50 are purchased, they are sold at the retail price. The wholesale price is 2 yuan cheaper per album than the retail price. If the class leader buys 1 photo album for each student, it will exactly use up $m$ yuan; but if 12 more photo albums are bought for the subject teachers, they can be settled at the wholesale price, and it will also cost exactly $m$ yuan. How many students are there in the class? What is the retail price of each photo album?
Three, 15. Let the class have $x$ students, and the retail price of each photo album be $y$ yuan. According to the problem, we have
$$
x y=(x+12)(y-2),
$$

and the integer $x$ satisfies $38<x<50$.
From (1), we get $12 y-2 x-24=0, y=\frac{x}{6}+2$. Therefore,
$$
x y=\frac{x^{2}}{6}+2 x \text {. }
$$

From (3) and $x y=m$ being an integer, we know that the integer $x$ must be a multiple of 6. From (2), $x$ can only be 42 or 48.
The corresponding $y$ values are 9 or 10, respectively.

However, $m<400$, so, $x=42, y=9$.