Final Answer:
Let
represent the number of boxes Susie sold. The equation to determine this is

Step-by-step explanation:
In this problem, we are dealing with the total number of chocolate boxes sold by Susie and her sister combined. Let
represent the number of boxes Susie sold. Since her sister sold 12 boxes, the total number of boxes sold is
. According to the information given, the total number of boxes sold is 28. Therefore, we can set up an equation to represent this situation:
![\[ S + 12 = 28 \]](https://img.qammunity.org/2024/formulas/geography/high-school/qloj5zzexn7zfofgb3keib6yg574v97h14.png)
Now, to find Susie's individual sales, we need to isolate
on one side of the equation. Subtracting 12 from both sides, we get:
![\[ S = 28 - 12 \]](https://img.qammunity.org/2024/formulas/geography/high-school/8jpfdr2u50vhythsiqttst09tjdkefrzwj.png)
Simplifying the expression, we find:
![\[ S = 16 \]](https://img.qammunity.org/2024/formulas/geography/high-school/lu7qly1wh7yldi9did75zf5awr66x9t6we.png)
So, Susie sold 16 boxes of chocolate. This solution is obtained by understanding the problem, setting up the appropriate equation, and then solving for the unknown variable through algebraic manipulation.