Mathematical formulation contains only what is important in a problem. One simple approach to find out is to try changing the problem a small piece at a time. As an example, let's start with one of the simplest problems. Every word and number shown in the applet is clickable. Numbers can be increased or decreased depending on whether you click a little to the right or to the left of their vertical center line.
Many formulations are possible for a single problem.
Nor is it important whether they owned, or bought, or had the goods in question. The nature of goods is also inessential. What is essential is that the problem is concerned with three quantities, two of which add up to the third one. One of the quantities is unknown. Firstly, the equation contains an arithmetic operation - addition.