Answer:
Diego paid $52 for 12 books.
Explanation:
If Mike bought 3 books for $13 that means that a book cost him $ 4.33
$ 13/3 = $ 4.33
And if Diego bought 12 books for the same price, how much did it cost?
An equation called "Rule of 3" is used:
1 book = $ 4.33
12 books = X
The X of the equation is cleared:
X = (12 books * $ 4.33) / 1
X = $ 52 for the 12 books Diego bought.