If 5 books cost d dollars, then the cost of one book can be found by dividing the total cost by 5:
Cost of one book = Total cost / Number of books = d dollars / 5 books = (d/5) dollars
To find how many books can be purchased for 2 dollars, we can divide 2 dollars by the cost of one book:
Number of books = Amount of money available / Cost of one book = 2 dollars / (d/5) dollars per book
Number of books = (2 x 5) / d
Number of books = 10/d
Therefore, the number of books that can be purchased for 2 dollars is 10 divided by the cost of 5 books, or 10/d