For example, 18 is a two-digit number.

The title says Nancy has a book, and the number of pages per page adds up to 8 13. How many pages are there in this book?

First of all, 1 to 9 has nine numbers.

Then there are two digits. 10-99 has 90*2= 180 digits.

There are 8 13 numbers in the book, so there are 8 13-9- 180=624 numbers left.

The next coded page numbers are all three digits, and 624 can be coded as 624/3=208 pages.

So this book has 100+208- 1=307 pages.