一道奇怪的prep题

Amy's grade was the 90th percentile of the 80grades for their 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 grade of the two classes combined?

ans: 85th

几乎没有看懂题的意思，请高人指点思路

谢谢！