A researcher for the EPA measured the amount of arsenic in the water near a sewage treatment plant. Over 5 days, he took $\displaystyle {n}={20}$ measurements (in ppb). His sample data is summarized in the ogive graph shown below.
[Graphs generated by this script: initPicture(17.5,67.5,-4,21);axes(5,5,"labels",2.5,1);marker="dot";line([22.5,0],[27.5,1]);line([27.5,1],[32.5,6]);line([32.5,6],[37.5,8]);line([37.5,8],[42.5,10]);line([42.5,10],[47.5,15]);line([47.5,15],[52.5,17]);line([52.5,17],[57.5,19]);line([57.5,19],[62.5,20]);text([47.5,-2],"Arsenic (ppb)","below");]
How many measurements taken in this time frame were less than $\displaystyle {50}$ ppb?

ans =