Final answer:
Sydney paid $32.50 for 10 books.
Step-by-step explanation:
To find out how much Sydney paid for the books, we need to know the cost of each book. Since Katie paid $13 for 4 books, we can divide $13 by 4 to find the cost of each book: $13 ÷ 4 = $3.25.
Since all books cost the same amount, Sydney will also pay $3.25 per book. Since she bought 10 books, we can multiply the cost per book by the number of books: $3.25 × 10 = $32.50.
Therefore, Sydney paid $32.50 for 10 books.