Hi Teagan,
This problem reminds you that if you increase a number by a certain percentage, you have to decrease it be a different percentage to get the same number.
An easy example is increasing a number by 100% which doubles it. So if you increase 10 by 100%, you get 20. To get back to 10, you have to decrease 20 by half of its value, or 50%.
So the easiest way to solve the problem, is to substitute real number. Let's choose an easy one . . . 100.
Marking up the cost by 28% means you have 28% of 100 to it. 28% of 100 is 28 and 100 + 28 = 128.
Now to get back to 100, you have to figure what percent of 128 is 28 and subtract it.
Let's set up a proportion: 28 is to 128 as X is to 100--> 28/128 = X/100
Cross multiply--> 2800 = 128X
Divide both sides by 128--> X = 21.875 percent