#12 - The number of North American children who are

[Administrator]

Complete Question Explanation

Must Be True—Numbers and Percentages. The correct answer choice is (C)

This is a Must Be True question with a stimulus
that does not contain a conclusion. But, this stimulus does provide
information about both the numbers and percentages of obese children,
and so you can end up with an answer that has either a number or a
percentage (though a numerical answer is more likely since the percentage
is fixed at a constant 15% in the stimulus).

The numerical information comes from the phrase, “The number of North
American children who are obese...is steadily increasing.” The percentage
information comes from the phrase, “children who are obese—that is,
who have more body fat than do 85 percent of North American children
their age.” The percentage information defines obese children as those
who fall into the top 15% among all children their age in terms of body
fat, and therefore the percentage is known to be constant. The numerical
information tells us that the actual number of obese children is increasing
(and since this is a Must Be True question we can accept that information
as accurate).

Answer choice (A): This answer is incorrect because there is no evidence
in the stimulus to support it. Although the stimulus mentioned four major
studies that apparently agreed about the increase in the number of obese
children, it would be an exaggeration to say that any time four major
studies produce similar results they must be accurate.

Answer choice (B): This answer proposes a causal reason for why the
number of obese children is growing. From the information in the stimulus
we cannot determine the cause of the rise in obesity, so answer choice (B)
is also wrong.

Answer choice (C): This is the correct answer. Consider the following
example:

     15 years ago—100 total children of similar age
     Number of obese children                15                     = 15%
     Number of non-obese children      85

Now, let us say that the number of obese children has risen to 150 children
today:

     Today
     Number of obese children               150

So far we have conformed to the information given in the stimulus:
the actual number of obese children is rising. However, although the
number of obese children has now risen to 150, the definition of obesity
(“more body fat than 85 percent of North American children”) remains
unchanged. Since this is the case, the 150 obese children today must still
comprise the top 15% of the total child population. Consequently, the
remaining 85% of non-obese children must now be 850:

     Today
     Number of non-obese children          850
     (150 is 15% of 1000, and thus 85% of 1000 is 850)

Answer choice (C) is fully supported because the stimulus provides
information about both the number and percentage of obese children. As
stated earlier, if the stimulus provides information about both the numbers
and percentages in a situation, then you can select any supported answer
choice that contains either numbers or percentages. Note the emphasis
on the word “supported.” In the obesity problem, LSAC could easily
have written an incorrect answer choice that says, “The number of North
American children who are not obese decreased over the past 15 years.”

Answer choice (D): This answer addresses “underweight” children, who
are neither defined nor discussed in the stimulus.

Answer choice (E): This answer is directly contradicted by the information
in the stimulus, which states that the incidence of obesity is definitionally
set at a constant 15%.

[Johnclem]

Could someone please explain this one ? - Please dumb it down as I don't get why the number of non obese had to increase. I remember learning in Class that two things could happen when numbers or ratios change. For example say we have a team of girls and boys soccer class, the only way the ratios of girls increase is if we have new girls join or have some of the boys leave. So I really was expecting the number of non obese to be lower.

1- the number of North American children who are obese is increasing.

* obese defined as those who have more fat than 85% of children their age.

Correct answer C) the number of North American children who are not obese increased.


Thanks
John

[Robert Carroll]

John,

The ratio can't change here. Obesity is defined in the stimulus as "more body fat than 85 percent of North American children their age." No matter how many children there are of that age, and no matter what their body fat, the obese children will always be those with more body fat than 85 percent of all children of the same age. The stimulus also says that the number of children who are obese has changed, not the ratio, so we know the number of children who have more body fat than 85 percent of children their age must also go up. That number corresponds to the top 15 percent of children. If the number of children in the top 15 percent went up, then the total number of children must also go up. That means the number of children in the bottom 85 percent must go up as well. This is why answer choice (C) is correct.

Robert Carroll

[mpoulson]

Hello,

I have read the explanation provided for Question 2 about obese children and the number increasing. However, I found the explanation insufficient to arrive at answer C. The text doesn't say that the percentage of obese children is rising or declining. However, answer C presumes that the percentage has remained constant which the text never says. How can we be sure that this is accurate without the information provided. Thank you.


- Micah

[Clay Cooper]

Hi Micah,

Thanks for your question.

The text of the stimulus does give us enough information to know that the percentage of kids who are obese has stayed constant; it does so by defining the term obese as applying to children who have more body fat than 85% of other children at their same age. In other words, we are told by the stimulus that, if you have more body fat than 85% of other kids your age, no matter how old you are, you are obese; if you don't, you aren't. Thus, these percentages are constant by definition (again, obese means top 15% by body fat); so if the number of obese kids has increased, then it must necessarily be the case that the number of non-obese kids has increased as well.

Keep working hard!

[mpoulson]

I'm not sure I understand where the stimulus says that the percentage of obese children is constant. I only see that the stimulus says "the # of NA chidlren who are obese...is steadily increasing". I know this alone doesn't mean obesity is rising or declining, but I am not sure how to come to the conclusion that "the # of NA children who are not obese increased over the past 15 years" without further information. Please explain where I went wrong.

- Micah

[Robert Carroll]

Micah,

The % is true by definition. That is how the stimulus defines obesity. See my above explanation about that.

Robert Carroll

[Johnclem]

Hello ,
I have been trying to make sense of this question for some time now. After reading your wonderful response I am still having trouble understanding . Reason being , I identify this question with some other ones. I.e ski slope injuries ( where as the percentage of ski injuries on the slope decreased , the percentage of other non slope injuries increased ). Or the sea water evaporation question ( where a greater number of oxy 16 left the water , thereby increasing oxy 18 in the water ).
I don't understand why the composition hasn't changed here as well


Thanks
-John

[Clay Cooper]

Hi John,

Thanks for your question.

It's hard to know exactly what you're asking without being able to refer to those other questions - can you name what section and numbers they are?

That said, it sounds like what those problems feature in the stimulus is a percentage composition of the total number that varies (e.g. if the percentage of total injuries accounted for by one type of injury goes down, the remaining categories' percentage must increase; same with oxy 16 and 18). In such problems, in short, the percentages change.

In this problem, the percentage of all kids who qualify as obese can never change. We are told that 15% of kids will always be obese when obese is defined as 'kids for whom 85% of kids weigh less.' That means that, no matter how many kids total there are, 15% of that total are obese and 85% are not. Once we know that, it is easy to see that if the number of non-obese kids has gone up, so must have the number of obese kids.

Does that help?

[Johnclem]

Hi Clay ,
Here are the test numbers for those problems that I thought were similar to this one :
PT 1(3)Q11
PT 2(2)Q16

Also, I think I'm getting it. So what I am understanding is in the pervious problems that I referenced the percentage group was not defined. But in this problem it is defined as the " 85% off all kids their age " ( and so when a percentage is defined for us we can't change it ) am I understanding this correctly ?

Here's an example of what I mean.: If I order a glass of half rum and half coke ( because I am an alcoholic) and the bartender increases the coke , she or he would also have to increase my rum, to maintain my half and half order.



Thanks
John