问题如下：
The following table shows the annual MSCI World Index total returns for 2004–2013.
The fourth quintile return for the MSCI World Index is closest to:
选项：
A. 20.65%.
B. 26.03%.
C. 27.37%.
解释：
B is correct.
Quintiles divide a distribution into fifths, with the fourth quintile occurring at the point at which 80% of the observations lie below it. The fourth quintile is equivalent to the 80th percentile. To find the yth percentile (Py), we first must determine its location. The formula for the location (Ly) of a yth percentile in an array with n entries sorted in ascending order is Ly = (n + 1) × (y/100). In this case, n = 10 and y = 80%, so L80 = (10 + 1) × (80/100) = 11 × 0.8 = 8.8. With the data arranged in ascending order (-40.33%, -5.02%, 9.57%, 10.02%, 12.34%, 15.25%, 16.54%, 20.65%, 27.37%, and 30.79%), the 8.8th position would be between the 8th and 9th entries, 20.65% and 27.37%, respectively. Using linear interpolation, P80 = X8 + (Ly – 8) × (X9 – X8), P80 = 20.65 + (8.8 – 8) × (27.37 – 20.65) = 20.65 + (0.8 × 6.72) = 20.65 + 5.38 = 26.03%
老师能不能解释一下插值法,也就是8·8位置的数为什么是0·8乘以两数相减