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?
72nd
80th
81st
85th
92nd
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%
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