A percentile example.
Data set: 1,3,5,6,7,8,9,10
Find the 75th percentile. i=(75/100)*8=6
Since i is an integer (6), the 75th percentile is the average of the numbers in the 6th and 7th positions.
The average of 8 and 9 is 8.5 and thus any value above 8.5 is in the upper 25% of the data.