Interquantile range (IQR) is represented by the gray box.
IQR = Q3 - Q1 //range for 50% middle values (of all values in the data)
thus Q1 is left side of the gray box, Q3 is right side of the gray box
Q1 = 8 //25% values are less or equal than Q1
Q3 = 18 //25% values are more or equal than Q3
answer: Q3 = 18
