7. To number the pages in a mathematics textbook, 390 digits were required. How many pages are in the mathematics textbook?
7. For writing the first nine pages, 9 digits are needed $(1,2,3,4,5,6,7,8,9)$.

For numbering pages from 10 to 99, 90 two-digit numbers are needed, which use $90 \times 2=180$ digits.

In the next step, we determine how many digits were used to write three-digit numbers $(390-180-9=201)$. Now we find out how many pages were written using three-digit numbers $(201: 3=67)$.

On the last step, we find the answer to the problem $(9+90+$ $+67=166)$

Answer: the mathematics textbook has 166 pages.

## EIGHTEENTH