Your sample data are discrete, so percentiles are not unique. However, we can smooth the data and create unique percentile values called smoothed empirical percentiles.
If you have
observations, then the
th smoothed empirical percentile is found by interpolating between the two data points that are around the
observation.
If
is not an integer, interpolate between the order statistics before and after the
Your data have
observations.
So the 60th smoothed empirical percentile is
where
is the
th order statistic.