percentile=成绩比他低的数/总成绩个数*100%
成绩比amy低的人数是80*90%=72，另外一个班19个比他高，81个比他低，加在一起比amy分低的人数是72+81=153
percentile=153/180约等85%

There's also the brute force method, which doesn't take that much time here.

Amy's grade is 90th percentile, meaning that she outperformed 90% of her classmates. That means that she outperformed 72 of her classmates.
She also outperformed 81 of the students in the other class (note: not 80, because Amy herself is not in the other class).

This means that she outperformed 72 + 81 = 153 of the combined 180 students in the two classes.
Since 153/180 = 0.85, she outperformed 85% of the students in the combined pool, putting her in the overall 85th percentile.

80*(1-90%) =8 →There are 8 people ahead of Amy in her class
Combined with another class, there are 19+8=27 people’s grades ahead of Amy’s.
# of people whose grades below Amy’s / total people of twp class → (100+80-27)/(100+80)=85%

percentile，从小往大数/从后往前数的名次/总数；或1-前面的人数/总数；或(后面的人数+1)/总数
在A班级，自己排72th，前面有8人后面有71人，总共是80人；B班级，前面有19人，后面有81人，总共100人（Amy不在这一百人里）
加起来前面有27人，后面有152人，自己排第153，percentile=1-27/180=153/180=85%

艾米的percentile=艾米在一群人中的名次/这群人的总数，比如这群人有10人，分数从小到大排下，1112334555，艾米4分，那么艾米的 percentile =艾米在一群人中的名次（6）/这群人的总数（10）=60%

the 90th percentile of the 80 grades该怎么理解哇

Percentile是百分比吗