The answer depends on how much Pedro spends on each book. You did not specify so let’s use an example... Let’s say that Pedro spends $5.00 a book simply multiply 5x4 to get $20. Pedro spends $20 on 4 books. Another example... Let’s say that Pedro spends $4.00 a book simply multiple 4x4 to get $16.00. Pedro spends $16.00 on 4 books.
