Since it's a cumulative frequency graph, the vertical axis indicates the number of people you have (ie 60).
As normal for graphing box and whisker plots, follow these steps:
1) Find the median
2) Find the lower quartile (find the no. in the middle of the median & lowest score)
3) Find the upper quartile (" " highest " ")
4) Draw the box indicating the lower & upper quartiles, median, then the line....
In this case, look on the vertical axis to find the median. There are 60 people, so the median is between the 30th and 31st number. If you go to 30.5 on the vertical axis, then go across, the median is 155cm.
Now, the lower quartile, is found between the median and the lowest number. ie between 30 and 1. Median of these set of numbers is between 15 & 16 = 15.5 like before, go to 15.5 on the vertical axis, go across and it will be 145cm.
Repeat for the upper quartile (this one has a trick - the upper quartile is between 45th and 46th numbers. From the graph, you can see the 45th number is 165cm, and the 46th number is 175cm. Average 165 and 175 = (165 + 175)/2 = 170)
