You should be talking about the code of the book, not the code. The back of each book is printed with the internationally unified book code ISBN, which acts as the ID card of the book. Each group of ISBN consists of 13, and the former 12 is divided into four groups, which are used to display the prefix code 978 of ISBN system and the regional language, publishing house, title and other information of books, and the last group of numbers is used to check whether the previous numbers are accurate. For example, the test number in ISBN978-7-56 17-3058-4 is 4, which is calculated by multiplying the first 12 of ISBN by 1 (from the left), multiplying the even number by 3, dividing by 10, and then using 6544. For the book number:1× (9+8+5+1+3+5)+3 × (7+7+6+7+0+8) =136,136 ÷ 647;.