This is a good one, thanks for posting.
If Amy is the 90th percentile in a class of 80 students, that means that she was number 72 out of 80 (calculation: .9 * 80). So, there were 8 scores above her.
Now combined, there are 180 students (80 + 100). Of those, 27 scored higher than Amy (the 8 we already calculated plus the 19 in the other class).
Thus, Amy was score number 153 out of 180, which is the 85% percentile (calculation: 153/180).
Hope this helps!
Gmat prep percentile
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There's a MUCH quicker way to find the answer to this question.
Amy was 81st %ile in one class and 90th %ile in the other class. Therefore, her OVERALL %ile must be between 81st and 90th.
Only one answer fits within that range: choose 85th.
Amy was 81st %ile in one class and 90th %ile in the other class. Therefore, her OVERALL %ile must be between 81st and 90th.
Only one answer fits within that range: choose 85th.
Stuart Kovinsky | Kaplan GMAT Faculty | Toronto
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.. I guessed it and unfortunately it was wrong 
