We could use this equation like this:
I spent 17.50 on two articles at the store, one of the articles cost me 15, how much did the other article (c) cost?
Then we have the equation:
15 + c = 17.50
Where we are adding the articles bought, equal to the total spent, we solve for c:
c = 17.50 - 15
c = 2.5
Then the unknown article costs 2.5 and then all the variables are known in the problem.