[求助]请教Quantive Methods题目

看书的时候遇到个问题，请不吝赐教，谢谢

07年的NOTE，L1  P145，第七题：

below are the returns on 20 industry groups of stocks over the past year:

12%,-3%,18%,9%,-5%,21%,2%,13%,28%,-14%,

31%,32%,5%,22%,-28%,7%,9%,12%,-17%,6%

B  what is the 40th percentile of this array  data?

answer: the location of the 40th percentile is (20+1)(40/100)=8.4.the 8th and 9th lowest returns are 6% and 7%,so the 40th percentile is 6+0.4(7-6)=6.4%

为什么是（20+1）呢，明明是20个data，好困惑，请老师指教！