book 1 ，reading 3，LOS 7.f: Calculate and interpret quartiles, quintiles, deciles, and percentiles ，
The formula for the position of the observation at a given percentile, y, with n data points
sorted in ascending order is : *Ly = (n + 1) y/100
is it correct ？
in my opinion ，the formulation should be：Ly=（n-1）*y/100 + 1，
