Answer:
3 books per month
Explanation:
In rate problems you divide one kind of unit by another so that the units come out the way you want them.
Here we want to know "books" per "month," or books/month.
We know "books," and we know "months," so plug those in:
That equals 3 books/month, and sounds about right. See how the units stay as a fraction? That's the key to these.
Take care.