7. An editor uses the digits $0 \sim 9$ to number the pages of a book. If a total of 636 digits were written, then the book has $\qquad$ pages.
$$
\text { Ni, 7. } 248 \text {. }
$$

Assuming the book has $x$ pages, then we have
$$
(x-99) \times 3+90 \times 2+9=636 \text {. }
$$

Solving for $x$ gives $x=248$.