list data in order: 20, 25, 30, 30, 31, 40, 41, 49
the IQR is the difference between Q3 and Q1
First find the median: Average the middle two numbers = 30.5
Now the median of the first half of the data = Q1 = (25 + 30)/2 = 27.5
the median of the second half of the data = Q3 = (40 + 41)/2 = 40.5
IQR = 40.5 - 27.5 = 13