Quintiles divide data into 5 equal parts, i.e. fifths. So 3rd quintile splits the data set into the lower 60% of values and the upper 40% of values. That’s where 60 comes from.
Formula: They use the formula Ly = (n + 1)(y) / (100) to determine the position of the yth quintile (where y represents the percentile, in this case 60 for the third quintile).
Calculation: Plugging in the values (n = 13 for the number of data points and y = 60 for the percentile), they get: L(60) = (14)(60) / (100) = 8.4.
Interpretation: This value (8.4) doesn’t directly correspond to a specific data point because it falls between the 8th and 9th positions (13.6% and 13.9%) in the ordered data set.