求助！PREP Practice Test 2里面的一道数学~！

升c小调

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, 92th


答案是D，上来第一道就卡在这了~90th表达的是什么意思呢？怎么算的哩？请牛人们帮忙~~~

升c小调

第一个班里是第72，第二个是第20，这两个加起来除以总数不对啊~~

eflet222

90th是说在班里有10%的人成绩超过他,所以他自己班里是8个人超过他,另外一个班19个人超过他并且没有和他重分的,就是180个人里有27人超过他,所以他在1-27/180=85th的位置

升c小调

90th是说在班里有10%的人成绩超过他,所以他自己班里是8个人超过他,另外一个班19个人超过他并且没有和他重分的,就是180个人里有27人超过他,所以他在1-27/180=85th的位置

-- by 会员 eflet222 (2010/8/3 15:53:22)

喔~理解错了~~~谢谢！！！！