The easiest way to think of these is to put them in terms of percentages. Obviously, if you are SAVING 25%, you are SPENDING 75%. So if she SPENDS 13.50, we can easily use the 75% in an equation. $13.50 = .75x, with x being what the shoes cost originally. The .75 indicates that she is spending 75% of what the original cost is. You don't have any examples to pick from, but this is the best way to explain what one of the equations could be. You could also base it on money saved, but the equation is not quite as neat.
