Make an ordered dataset, i.e. sort the data in ascending order:
120, 130, 150, 160, 170, 190 ,200
First find the median, the middle value.
If the number of data in the set is even, take the mean of the two middle numbers, in this case, 160 is the median.
120, 130, 150, 160, 170, 190 ,200
Take the lower and upper halves of the data set and repeat the same procedure to get the quartiles.
120, 130, 150, 160
The first quartile is the mean between 130 and 150, thus 140.
160, 170, 190 ,200
The third quartile is the mean between 170 and 190, thus 180.
The interquartile range equals
180-140 = $40
Note: there may be other variants of calculating the quartiles. Check your class notes or textbook.
