A book sold 31,100 copies in its first month of release. Suppose this represents 7.8% of the number of copies sold to date. How many copies have been sold to date?
Round your answer to the nearest whole number.

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
There has been 398,718 copies sold to date.