For this case we propose a rule of three:
31700 ------------> 8.8%
x ----------------------> 100%
Where "x" represents the number of copies sold to date.
![x = \frac {100 * 31700} {8.8}\\x = \frac {3170000} {8.8}\\x = 360,227.273](https://img.qammunity.org/2020/formulas/mathematics/middle-school/cveosa11etij4lloo4m0aylo3qazt4peqi.png)
Thus, to date 360,227 copies of the book have been sold.
ANswer:
To date, 360,227 copies of the book have been sold.