明天就考啦···prep的两道题··谢谢~

小娟子nana

if x is equal to the sum of the even integers from 40 to 60 , inclusive , and y is the number of even integers from 40 to 60 , inclusive, what is the value of x+y

答案是561
这个是怎样算出来的？

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 classes combined?

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

答案是D
百度的答案（在 Amy 班那 80 个分数中，90% 比她的分数低。所以有 72 个分数比她的低。在那 100 人的班里，有 81 个比 Amy 的低。（注意：是 81 而不是 80，因为 Amy 不在这个班里面。）两班合起来就有 180 个学生，其中 72 + 81 = 153 个的分数比 Amy 的低。153/180 = 85%，所以 Amy 在这 180 人当中的 percentile 是 85。）
但我不明白“Amy's grade was the 90th percentile of the 80 grades for her class. ”为什么可以理解为“在 Amy 班那 80 个分数中，90% 比她的分数低。”