Final answer:
The subject of this question is Mathematics and the grade level is Middle School. Serena's book collection contains 23 fiction books and 4 non-fiction books in a part to part or part to whole ratio of 23:4.
Step-by-step explanation:
The subject of this question is Mathematics and the grade level is Middle School.
To answer the question, the given information states that Serena's book collection contains 23 fiction books and 4 non-fiction books. The ratio between the fiction books and the non-fiction books is part to part or part to whole ratio. This means that for every part of fiction books, there is a corresponding part of non-fiction books. In this case, the ratio of fiction books to non-fiction books is 23:4.