How many pages are there in the book?

Ratio of the read book $=\frac{7}{9}$

Ratio of the unread book $=1-\frac{7}{9}$

$=\frac{2}{9}$

Let x be the total number of pages in the book.

Thus, we have:

$\frac{2}{9} \times x=40$

$\Rightarrow x=40 \div \frac{2}{9}$

$=40 \times \frac{9}{2}$

$=\frac{40}{1} \times \frac{9}{2}$

$=\frac{40 \times 9}{1 \times 2}$

$=\frac{360}{2}$

$=180$

Hence, the total number of pages in the book is 180.