They started with 115 apples, and ate 8 apples each day.
Thus our expression for that would be
[tex]115-8x[/tex]
Now, we want to find after how many days they had 19 apples left. Thus, let's set the expression equal to 19 and solve for x.
[tex]115 - 8x = 19[/tex]
[tex]96 = 8x[/tex]
[tex]\boxed{x = \frac{96}{8} = 12}[/tex]
