Prof Liu gave the same quiz to the

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Prof Liu gave the same quiz to the

by NandishSS » Mon Dec 19, 2016 6:05 am
Prof Liu gave the same quiz to the students in her morning class and in her afternoon class. the average score for the two classes combined was 84. which class had more students?

(1) The average score for the students in the morning class was 80
(2) the average score for the students in the afternoon class was 86

OA:C

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by GMATGuruNY » Mon Dec 19, 2016 6:21 am
NandishSS wrote:Prof Liu gave the same quiz to the students in her morning class and in her afternoon class. the average score for the two classes combined was 84. which class had more students?

(1) The average score for the students in the morning class was 80
(2) the average score for the students in the afternoon class was 86

Statement 1:

Case 1: Average score for the afternoon class was 86.
Here, the average for the MIXTURE of the two classes (84) is closer to the average for the afternoon class (86) than to the average for the morning class (80).
Since the average for the mixture is weighted toward the average for the afternoon class, the AFTERNOON class had more students.

Case 2: Average score for the afternoon class was 100.
Here, the average for the MIXTURE of the two classes (84) is closer to the average for the morning class (80) than to the average for the afternoon class (100).
Since the average for the mixture is weighted toward the average for the morning class, the MORNING class had more students.

INSUFFICIENT.

Statement 2:
Case 1 also satisfies Statement 2.
In Case 1, the AFTERNOON class had more students.

Case 3: Average score for the morning class was 83.
Here, the average for the MIXTURE of the two classes (84) is closer to the average for the morning class (83) than to the average for the afternoon class (86).
Since the average for the mixture is weighted toward the average for the morning class, the MORNING class had more students.

INSUFFICIENT.

Statements combined:
Only Case 1 satisfies both statements.
In Case, 1, the afternoon class had more students.
SUFFICIENT.

The correct answer is C.
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by [email protected] » Mon Dec 19, 2016 7:27 am
Hi NandishSS,

This DS question is a great 'concept' question - if you recognize the concepts involved, then you don't have to do any math (however, you can TEST VALUES to get to the correct answer).

We're told that two classes (a morning class and an afternoon class) took a test and that the overall average for both classes was 84. We're asked which class had MORE students.

1) The average score for the students in the morning class was 80

Since the morning class had a score that was LESS than the overall average, the afternoon class must have an average that is GREATER than the overall average.

IF the average of the afternoon class was 88, then the two classes have the SAME size.
IF the average of the afternoon class was GREATER than 88, then the morning class is GREATER in size.
IF the average of the afternoon class was BETWEEN 84 and 88, then the afternoon class is GREATER in size.
Fact 1 is INSUFFICIENT

2) the average score for the students in the afternoon class was 86

This Fact is based on the same general idea as Fact 1...

Since the afternoon class had a score that was GREATER than the overall average, the morning class must have an average that is LESS than the overall average.

IF the average of the morning class was 82, then the two classes have the SAME size.
IF the average of the morning class was LESS than 82, then the afternoon class is GREATER in size.
IF the average of the morning class was BETWEEN 82 and 84, then the morning class is GREATER in size.
Fact 2 is INSUFFICIENT

Combined, we know the average of the two classes AND we know the overall average, so we can determine the ratio of the two class sizes (via the weighted average formula or allegation) and we can determine that the size of the afternoon class is larger.
Combined, SUFFICIENT.

Final Answer: C

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by Jay@ManhattanReview » Mon Dec 19, 2016 11:34 pm
NandishSS wrote:Prof Liu gave the same quiz to the students in her morning class and in her afternoon class. the average score for the two classes combined was 84. which class had more students?

(1) The average score for the students in the morning class was 80
(2) the average score for the students in the afternoon class was 86

OA:C

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Though you got the answer to the question with two great explanations.

I add 2% to this.

Two things to remember:
  • 1. If the combined average of two groups is the simple average of group's individual averages, we can conclude that the number of members in the groups are equal.

    2. If the combined average of two groups is NOT the simple average of group's individual averages, we can conclude that the number of members in the groups are NOT equal. Moreover, if the combined average of the groups, say group A and group B, is closer to one group than the other, say it is closer to group B than to group A, we can conclude that number of members in the former group, here group B, have more members than the later, here group A.
Coming to the question:

Given that the combined average of two groups = 84;

Average of group A (morning class) = 80;

Average of group B (afternoon class) = 86;

We see that 86 is closer to 84 than 80 is to 84, thus, group B (afternoon class) must have more students.

Hope this helps!

-Jay

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by Scott@TargetTestPrep » Wed Dec 21, 2016 9:00 am
NandishSS wrote:Prof Liu gave the same quiz to the students in her morning class and in her afternoon class. the average score for the two classes combined was 84. which class had more students?

(1) The average score for the students in the morning class was 80
(2) the average score for the students in the afternoon class was 86

OA:C
This is a great question testing the foundations of weighted averages. We must remember that the weighted average will always be closer to the average of the group that has a larger quantity. Let's test this theory with a simple example.

Let's say there are 5 women in a room with an average age of 40 and 3 men in a room with an average age of 20. The weighted average age of all the people in the room is as follows:

(5 x 40) + (3 x 20)/(5 + 3) = (200 + 60)/8 = 260/8 = 32.5 years old

Notice that the weighted average of the ages of all people in the room is closer to 40 (the average age of the 5 women) than it is to 20 (the average age of the 3 men). The reason for this is that there are a greater number of women than there are men.

Let's apply this principle to the above problem.

We are given that the combined average score for the morning and afternoon class is 84. We need to determine which class had more students.

Statement One Alone:

The average score for the students in the morning class was 80.

Since we do not have any information regarding the average score of the afternoon class, we do not have enough information to answer the question. Statement one alone is not sufficient to answer the question. We can eliminate answer choices A and D.

Statement Two Alone:

The average score for the students in the afternoon class was 86.

Since we do not have any information regarding the average score of the morning class, we do not have enough information to answer the question. Statement two alone is not sufficient to answer the question. We can eliminate answer choice B.

Statements One and Two Together:

Using the information from statements one and two, we see that the average score of the morning class was 80 and that the average score from the afternoon class was 86. Using the example provided above, we know that since the average score from the afternoon class is closer to the combined (i.e., weighted) average of 84, there are more students in the afternoon class.

Answer: C

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