The number of North American children who are obese—that is, who have more body fat than do 85 percent of North Ameri...

I had a hard time wrapping my head around this, but I found that the key is in getting a full picture on what the stimulus is saying. "The number of North American children who are obese—that is, who have more body fat than do 85 percent of North American children their age—is steadily increasing." What’s important to note is that their statement is based on a study done using the ratio of 85/15 to determine who is (the 15%) and who is not (the 85%) obese during different periods over the course of their study. Since they concluded from the study that the number of obese kids (the 15%) are steadily increasing, it must be true that the number of non-obese kids (the 85%) must have also increased as that would be the only way to maintain the 85/15 (not obese/obese) percent ratio.