Let's just take a look at the first two points plotted, (0,100) and (1, 80). The difference between 100 and 80 is 20, but what percent of 100 is 20? Well, since any percent x% can be represented as a fraction in the form [tex] \frac{x}{100} [/tex], and the the ratio of 20 to 100 is already [tex] \frac{20}{100} [/tex], our answer is a 20% decrease, which means that you're taking 100 - 20 = 80% of the previous area at each point.
Edit: Since I misread the question, I want to reframe my previous explanation instead in terms of what percent of the area of the previous point is left.
Looking again at the areas plotted, what percent of 100 sq. cm is 80 sq. cm? We want to translate the question into math, and our first indicator of where to start is the phrase "what percent of 100 sq. cm." When we talk about taking a fraction or percent of another quantity, we're usually referring to multiplying that fraction by that quantity. In this case, we have our unknown percent, [tex] \frac{x}{100}[/tex], multiplied by 100, which, of course, just leaves you with x. And we're told that this quantity is equal to 80, so we have our answer. x = 80, so we're going to be taking 80% of the area each step.
