Well, is just their difference, let's first convert the mixed fraction to "improper" and then subtract.
Using elimination, you can do
4x+3y-(4x-2y)=3-(18)
5y=-15
y=-3
Substitute it back into original equation:
4x+6=18
4x=12
x=3
<h2>
So, the answer is <u><em>
x=3, y=-3</em></u></h2>
X - (X)(.20) = $120
X (1 - .20) = $120
X (.80) = $120
X = $150
Original price of the boots was $150.