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by selango » Wed May 26, 2010 8:04 am
Amy's grade was the 90th percentile of the 80 grades for her class. Of the 100 grades from another class, 19 were higher than Amy's and the rest were lower. If no other grade was the same as Amy's grade, then Amy's grade was what percentile of the grades of the two combined?

A. 72nd
B. 80th
C. 81st
D. 85th
E. 92nd


OA D
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by sk818020 » Wed May 26, 2010 9:13 am
If Amy is in the top 90 percent of her class of 80, then 8 students are in front of her because,

80*.9=72

Amy's spot is 72 so there are 8 people infront of her. We also know that there are 19 people ahead of her school in the other class.

If you combine the classes there will be 8+19=27 people in front of her. To figure out her % of the combined group, devide her spot by the total number of people in the combined class.

27/180=9/60=3/20

To determine the % we need to get the denominator to 100 or close to 100.

(5/5)(3/20)=15/100

100-15=85

So Amy is in the top 85% of the combined class.

Hope this helps,

Jared
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by gmatmachoman » Wed May 26, 2010 9:17 am
selango wrote:Amy's grade was the 90th percentile of the 80 grades for her class. Of the 100 grades from another class, 19 were higher than Amy's and the rest were lower. If no other grade was the same as Amy's grade, then Amy's grade was what percentile of the grades of the two combined?

A. 72nd
B. 80th
C. 81st
D. 85th
E. 92nd


OA D
It 's a weighted average concept!!

If 19 students are higher than Amy's grade, it means if Amy is there in that class, she would have scored 81 percentile!!

so that leads to :
[80 * 90 + 100 * 81}/ (80+100)

= 85 !! PickD
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by indiantiger » Wed May 26, 2010 12:17 pm
what we need to find out is: above how many grades is Amy's grade in the total of her class + the other class.

total = 100+80 = 180
Amy's grade is above 90*80/100 in her class
= 72

Amy's grade has 19 grades above it in the other class: That makes her grade above 81 grades

so combined she is above 72+81 grades = 153

To find the combined %entile = 153/180 = .85 or 85th (D)
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by Patrick_GMATFix » Wed May 26, 2010 12:36 pm
Percentile is the percentage of values that fall below. Since Amy is in the 90th percentile in her class and in the 81st percentile in the other class (better than 81 of the 100 grades), her percentile rating when both classes are combined must be somewhere between 90th and 81st. Only one answer could be correct. D is the answer

This is GMATPrep question 1386. You can practice similar questions if you have access to the Solutions Engine drill generator by selecting topic="Statistics" and difficulty="400-500"

Best of luck,
-Patrick
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