The pth percentile of a list is the number such that at least p% of the values in the list are no larger than it. So the lower quartile is the 25th percentile and the median is the 50th percentile. One definition used to give percentiles, is that the p’th percentile is the 100/p*(n+1)’th observation. For example, with 7 observations, the 25th percentile is the 100/25*8 = 2nd observation in the sorted list.
Similarly, the 20th percentile = 100/20*8 = 1.6th observation.