`{:(X,"Frequency(f)","Cumulative Frequency"),(3," "2," "2),(6," "9," "11),(7," "13," "24),(9," "17," "41),(10," "10," "51),(11," "14," "65),(13," "15," "80),(," "N = 80,):}`

Here, N = 80

`therefore Q_(1) = ((N)/(4))`th observation = 20th observation

`therefore Q_(1) = 7` (as 20th items lies in the class having 24 as cumulative frequency)

`therefore Q_(3) = 3((N)/(4))`th observation = 60th observation

`therefore Q-(3) = 11` (as 60th item lies in the class having 65 as cumulative frequency)

Semi-inter quartile range, `(QD) = (Q_(3)-Q_(1))/(2) = (11 - 7)/(2) = 2`.