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.
请问答案为什么是 B啊  用书上的公式  （n+1）X% 应该是C呀
谢谢了 谁能帮我解答下