percentile matching

Why is it in some questions that ask for percentile matching at a certain percentage based on a set of data points.

For example percentile matching at 90%
from the following numbers 2, 3, 3, 6, 7, 7, 8, 8, 11, 12 (n numbers)

The solutions use a couple different methods, they either:
1. (.9)n
2. (.9)(n+1)
3. [(.9)n]rounded to the next whole number


what is the rule with when to use each one?

thanks!


percentile matching