Its quite simple actually. Because Sabah has $450, that is the total amount. remember that each book costs about $75 dollars, and that four of them can be gotten for free. Therefore use (b= (450/75) + 4).
450 is the total amount of money and it is divided by 75 to get the total number of books possible under that amount per book. The reason four is added is that it is stated that four of the books necessary are to be taken from a library for free. Therefore, when solved, b is equal to 10
