two fifths of the books were put on the shelf. what fraction of the books was not placed on the shelf

Answer 1

Since you are talking about a whole collection of books, we can assume that 5/5 or 1 whole would represent all of the books. If you have already put 2/5 onto the shelf, then you are looking for what would be left when taking 5/5 (your total) and subtracting 2/5 (the books already on the shelf).