For this problem you can create a proportion.
Since 31,100 copies is 7.8% of the total copies spoke up to date you can make this proportion: 
31,100.     7.8%
——— =.  ———
    x.           100%
Now you cross multiply and you will get 
7.8x=3,110,000
After this you solve for x by isolating the variable. 
7.8x/7.8. 3,110,000/7.8
x=398,717.949
Since the question asks you to round to the nearest whole number which is 7 then the final answer would be 398,718
 Answer: There has been 398,718 copies sold to date.