You have an empty box which is 1,800 grams. After you put the books in, it's 9 kilograms. There are 9,000 grams in 9 kilograms, and 1,800 grams were already there before you put the books.. so just do 9,000 - 1,800 which equals 7,200. Then divide 7,200 by 4, because there are 4 books with equal mass.
