IQR is the difference between Q3 and Q1
64, 68, 70, 71, 72
Q2 = 70
Q1 is the median of the lower half. That will be the average of 64 & 68.
(64+68)/2= 66
Q3 is the median of the upper half. That will be the average of 71 & 72.
(71+72)/2 = 71.5
IQR= 71.5-66 = 5.5