Mock afternoon #20求教 afternoon, following, normal

xtw89

The following ten observations are a sample drawn from a normal population: 25, 20, 18, -5, 35, 21, -11, 8, 20, and 9. The fourth quintile (80th percentile) of the sample is closest to:
A. 8.0.
B. 21.0.
C. 24.2.

为什么答案不是  C？ 不是应该用 (n+1) (Y/100) 这个公式吗？