Explanation:
the first 9 pages have 1 digit (1 to 9).
the next 90 pages have 2 digits (10 to 99).
that is in total
9×1 + 90×2 = 189 digits.
that leaves us
459 - 189 = 270 digits for pages with 3 digit page numbers (100 and up).
there are 900 pages with 3 digit page numbers = 2700 digits, which is larger than 270.
so, the solution must be in this interval (there are no pages with 4 digit page numbers like 1000).
after how many pages with 3 digit pages numbers do we have a total of 270 digits ?
270/3 = 90 pages.
so, we have 9 + 90 + 90 = 189 pages in the book.