study 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

Being in 90% percentile out of 80 grades means Amy was 73rd (from the least grade) in the class (as long as nobody had Amy's grades), she outscored 72.

In another class her grade would rank her 82 (from the least grade out of 101, 100+Amy), 81 had the grades lower than Amy.

So in combined classes she outscored 72+81=153. There are 180 students 153/180=0.85

Answer: D (85%)

Generally being in x% means that x% has the grades less than you.

Amy in 90% --> she outscored 90% of of 80=72 (Amy is 73rd).

Of the 100 grades from another class, 19 were higher than Amy's, and the rest were lower --> rest is 81, hence Amy outscored 81.

So out of total 80+100=180, Amy outscored 72+81=153. Hence she is in 153/180=85%.

How were you able to understand that in the first class there were 80 students, and in the second one there were 101 and not 100?