Thanks for the question! So the trick with this one is to look carefully at what it means to be “obese” for the purposes of this stimulus. Obesity is defined as having more body fat than 85% of North American children of the same age. So what qualifies someone as obese? Well, if there’s 100 North American kids of the same age, they had to have more body fat than 85 of them. If there’s 1000, they have to have more body fat than 850.
So the rest of the stimulus tells us that there are more and more obese North American children. Well, if the number of obese children is going up, then the total number of children has to go up too! Because it’s a constant proportion of obese to non-obese. Let’s say there’s 15 total obese children at some point in time. Well, that means that there’s 100 total children, since obese children are going to make up 100 - 85 = 15% of the population, and 85 non-obese children. So now let’s say there’s 30 obese children. Well, the obese children are always 15% of the total relevant group! So now there have to be 200 kids total, and 170 non-obese kids. So if the obese children doubled, the North American children the same age had to have doubled too, and the non-obese kids also have to double. And that’s what (C) tells us, that if there’s more obese kids, there also have to be more non-obese kids to keep the proportions the same.
Hope this helps! Feel free to ask any other questions that you might have.
andreaskormusisJuly 30, 2020
I sort of understand. Is it because it says the number of children who are obese is increasing as opposed to saying that the percentage of children is increasing? Like your example of 85 skinny kids and 15 obese kids, if I were to say that the percentage went from 15% to 20% we'd be assuming that it went from 15 obese kids to 20. Whereas saying the number of children with a constant percentage implies that the skinny kids must also increase with the obese kids. Does that make sense?
shunheJuly 31, 2020
Yes, it's exactly like that! If the proportion stays constant and one of the parts changes, then the overall number has to change. And that's just like how if the overall number stays the same, and one of the parts changed, then the proportion has to change.
