In a freshman biochemistry class at Newton University, the teacher

Question

In a freshman biochemistry class at Newton University, the teacher assigns the class only ‘A’s, ‘B’s’ or ‘C’s. The average on the semester final for the class of 2012 was five points lower than that of the class of 2011. Therefore, the percent of students who received ‘C’s was greater in 2012 than in 2011.

Which of the following, if true, suggests the conclusion above is not necessarily valid?

There was a greater number of students in the 2011 class than in the 2012 class.
The percent of students who received ‘A’s in 2011 was less than in 2012.
Five more students received ‘A’s in 2012 than in 2011.
The same number of students received ‘B’s in 2011 as in 2012.
The percent of students who received ‘B’s was greater in 2012 than in 2011.

KarishmaB · Accepted Answer

There are three grades A, B and C.
The average score was 5 points lower in 2012 than in 2011

Conclusion: Percent of students who got Cs was greater in 2012 than in 2011

Because the average score is lower, the assumption is that more percentage of people got Cs. So it doesn't matter how many people are there in either year. We are talking about the average score and the percentage of students.

We want to weaken the conclusion.

(A) There was a greater number of students in the 2011 class than in the 2012 class.

This, as we discussed above, is irrelevant.

(B) The percent of students who received ‘A’s in 2011 was less than in 2012.

Percent of students who got A in 2012 > Percent of students who got A in 2011
Since average score is lower, more percent of students who got A means more percent of students got C.
Grade A might be given to students getting more than average (simply speaking). If there are more such people, then there will be more people who will get less than average (and hence grade C) to get to the average.

Say these are the scores of 5 people: 
3 4 4 4 5

If you have more people getting 5, a higher score, you will need more people to get 3 too to make up the average of 4:
3 3 4 5 5

So option (B) makes our conclusion more likely, not less.

(C) Five more students received ‘A’s in 2012 than in 2011.

Again, numbers are irrelevant.

(D) The same number of students received ‘B’s in 2011 as in 2012.

Numbers are irrelevant.

(E) The percent of students who received ‘B’s was greater in 2012 than in 2011

Even though the average is lower, more percent of students received B's. It is possible that the lower score still gives a B. 
IF more people get B (around average), fewer people will get both A's and C's.

Say initially, the scores looked like this: 3 3 4 5 5

If students at average increase, it may look like this: 3 4 4 4 5

Hence percentage of C's may actually decrease.

This weakens our conclusion.

Answer (E)

SonaliT · Answer

Can anyone shed some light on why the OA is E?

Temurkhon · Answer

The average score relates to weighted mean concept:

A*x+B*y+C*z/x+y+z, where x,y,z is the percent of every type of students (i.e A,B,C). If z is more we get increase of average, if x OR y is more we get decrease

Conclusion says that in 2012 av. score 5 point less than in 2011 because more C. What is the alternative reason?

A. There was a greater number of students in the 2011 class than in the 2012 class. (It doesn't matter, we are interested only in percemt of every type of students)

B. The percent of students who received ‘A’s in 2011 was less than in 2012. (If so, av. score in 2012 could be higher)

C. Five more students received ‘A’s in 2012 than in 2011. (We do not know the percentage, so out of scope)

D.The same number of students received ‘B’s in 2011 as in 2012. (Again, no percentage, out of scope)

E. The percent of students who received ‘B’s was greater in 2012 than in 2011 (It fits the concept)

E

ChiGMAT · Answer

The first four options can be eliminated based on the reasons given by Temurkhon.

Option E is correct because if more students received a B as compared to the previous year, then the average could go down. So, that is another possible reason instead of the conclusion given in the argument.

Hope this helps

Aves · Answer

Is this kind of question appear in CR when I take the real test?

I doubt it

KyleWiddison · Answer

I'm responding to a PM on this one.

The question is a weakening question because it is looking for information that would make the conclusion less valid. It's not typical of a question you would see on the real test, but it does highlight logic errors that you will see on the real test. The test loves to transition between number and percentage on the real test, and this question does a good job of testing that issue.

If you can understand the logic of the argument and can reason through the answers to see which one would weaken the conclusion, you have skills that would serve you well on the exam...

KW

BicepBrachii · Answer

Came for the title of the problem - nicely done!

KyleWiddison · Answer

I didn't even see the title the first time around. That is classic.

akadmin · Answer

Why is it not a typical GMAT Question? It looks like a typical one to me . My following question is if its a 700+ Question. I think its 600 level Question. Please advice

KyleWiddison · Answer

What I'm getting at here is that it's from a prep company, not the actual GMAT. We do our best to approximate the GMAT, but it's not the same. This particular problem points out an issue that you will see tested on the GMAT, but the GMAT is typically more subtle than this...

As far as the difficulty, it's super hard to determine. Based on the performance of the people taking this one I would probably peg this as a high 600 level...

KW

As far as the difficulty, it's super hard to determine. Based on the performance of the people taking this one I would probably peg this as a high 600 level...

KW

mvictor · Answer

woah..question from hell...
we speak about absolute values, thus, we can eliminate C and D right away, as we do not know what portion of the whole - the numbers given represent.
A - doesn't tell much. for ex. in 2011 - 100 students, and in 2012 - 10 students. for ex. in 2011 - 10 students got C, and in 2012 - 2 student got C. the conclusion still hold true, as in 2012 - 20% got C, while in 2011 - 10% only.
B - if in 2011 the % of A was less than in 2012, then it means that in 2012 more C's would have been, otherwise a drop of 5 points could not have been explained.
E - the only one that weakens the conclusion. for example. in 2011 we had 100 students, all got A. in 2012, we got 100 students, 90 got A, while 10 got B. we clearly see that the average would drop, while no C's at all were registered.

popovarseniy · Answer

I find this question specific to countries that have the system that uses letters. How can I understand the correlation between letters and numbers?

Posted from my mobile device

Gmatprep550 · Answer

Hi  popovarseniy

I don't think one need to go in depth to find exact correlation between letters and numbers as this is neither a quant question nor they are asking you answer in accurate numbers, You just have to assume that higher number student will get A and lower will C.

Hope it helps !

Thanks !

ShankSouljaBoi · Answer

B and E both sound same to me. If A decrases, B or C could be manipulated accordingly. Please explain the quant behind both the options.

Regards

DavidTutorexamPAL · Answer

Let's say there are 3 students in the class. 
E tells us that is 2012, more received B's: so, for example we could have 3 B's in 2012 and 3 A's in 2011. Higher average in 2011, no change in % of C's. Conclusion not valid! 
B tells us in 2012 we had more A's. But since we know the  overall average  in 2012 is lower, there  must  be more c's: for example: 2012 - 1 A, 2 C's, 2011 - 3 B's. Average higher in 2011, conclusion still valid... not our answer

agar123 · Answer

Hi Karishma
Can you please explain why the number of people is irrelevant because if I try to prethink then in this passage we are talking about average. so average of 2012 is 5 points lower than 2011. the average can be lower in the following cases:

1. No. of people can be more in 2012.
2. people who have received grade A are more in 2011 class.
3. people who have received grade B or C are more in 2012 class as compared to 2011 class.
Although selected the correct answer, but my reasoning was option E was matching with my prethinking option 3. Am I right in my prethinking? After reading your post, I was not being able to understand why numbers are irrelevant here. Can you please comment?

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In a freshman biochemistry class at Newton University, the teacher

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