Box plot allows us to determine the mean, the largest or the lowest number in the data set as well as the quartiles in the data. In this case, from the given figure, median is equal to 18, the range is equal to 21-15 equal to 6. The 25th percentile is equal to17 while the 75th is equal to 20. IQR then is equal to 20-17 equal to 3.