For the moment, let us pretend that, in fact, someone else went to the store in your stead. A small child, perhaps, running an errand for you.
You know you gave them fifty gald before they left for the store. They can tell you how many apples they bought, and how much change they have left, but they don't remember how much each apple cost. Suppose, also, that you have long since left the town behind, but would like to know whether your errand-runner was able to get a good price for the apples.
[ rattling it off ] The problem could be represented as 50 - 20 = 6 * a, where 20 is the change they have left and 6 is the number of apples. The a represents the apple's cost.