Well, you are trying to simplify the numbers and the equation. You are trying to get the smallest possible number or solution. A difference between the two is that polynomials have factors and variables. Numbers are just numbers. With a polynomial, you have to combine like terms and try to reduce the length of the equation. You may not always fully solve a polynomial. With numbers, you probably will get an exact numeral answer.
So each book is an average of 2.8 cm so 120 divided by each book=number of books 120/2.8=42.85 round down since you can't fit .85 of a book on a shelf the answer is 42 books