Let the Addison's candy count be a and Ronny's be r.
Then a = r - 15. This is an equation.
If you had a second equation, you could replace a with the expression (r - 15), which in itself is not an equation.
Let Ronny's candy be x and Addison's candy be y. Then it would read y=x-15.
This is an equation because this is saying that Ronny's amount of candy minus 15 is equal to amount of candy that Addison has. Once there is a useful equal then it is equation.
