Solution:
We know that,
Number of pages in the book \[=\text{ }85\]
Number of possible outcomes \[=\text{ }n\left( S \right)\text{ }=\text{ }85\]
Out of \[85\]pages, pages that sum up to \[8\text{ }=\text{ }\left\{ 8,\text{ }17,\text{ }26,\text{ }35,\text{ }44,\text{ }53,\text{ }62,\text{ }71,\text{ }80 \right\}\]
So, pages that sum up to \[8\text{ }=\text{ }n\left( E \right)\text{ }=\text{ }9\]
Hence, probability of choosing a page with the sum of digits on the page equals \[8\text{ }=\text{ }n\left( E \right)/\text{ }n\left( S \right)\text{ }=\text{ }9/85\]