You can put this solution on YOUR website!
Percentage calculations are always done against the original..not the final answer, so lets call the original price x.
20% discount from x is (20/100)x or 0.2x. This discount is taken off the original, x, so we have x-0.2x which is 0.8x and this is the sales price of 12590.
So 0.8x = 12590
x = 12590/0.8
--> x, the original price = $15737.50
Hope this explains it simply enough for you?