They need to cover expense of $13200. If printing fee is $25 and they sell the books for $40 each, then that means when selling a book they cover expense of printing plus they earn $15 per book.

So if they earn $15 per book that means they have to sell `13200/15=880` books to break even.

If you want to solve this by using equation the this would look like this:

expenses: $13200 + $25`x` where `x` is number of books sold

earnings: $40`x`

Equation:

`13200+25x=40x`

`13200=15x`

`x=880`