To evaluate the data set shown we need to rearrange the sets from smallest to the largest
5,7,10,10,12,12,12,13,15,17,18,18,18,19,20,20
thus the:
first quarter, Q1 will be:
(10+7)/2
17/2
=8.5
second, Median
(13+15)/2
=28/2
=14
Third quarter, Q3:
(19+20)/2
=39/2
=19.5
Thus you can use the values obtained to mark the number line.