gdk800 wrote:

Angela’s grade was in the 90th percentile out of 80 grades in her class. In another class of 100 students there were 19 grades higher than Angela’s. If nobody had Angela’s grade, then Angela was what percentile of the two classes combined?

a. 72

b. 80

c. 81

d. 85

e. 92

She is in 90th percentile in one class and 81st in the other. We need to find her 'average percentile'. The average will lie between 81 and 90 so it has to be 85!

If we had closer answer options e.g. 84, 86 etc, we would need to calculate the weighted average.

Using weights as 80 and 100 respectively, you can find their average using weighted average formula which is as given below:

\(C_{avg} = \frac{C_1*W_1 + C_2 * W_2}{W_1 + W_2}\)

\(C_{avg} = \frac{90*80 + 81*100}{180}\) = 85

In case you are not comfortable with weighted averages, check this discussion:

http://gmatclub.com/forum/tough-ds-105651.html#p828579and get back if there is a doubt.

This formula helped tremendously.