A researcher for the EPA measured the amount of arsenic in the water near a sewage treatment plant. Over 12 days, he took n = 40 measurements (in ppb). His sample data is summarized in the frequency distribution polygon shown below. 12 10 - 30 35 40 45 50 55 60 65 70 75 80 45 50 55 60 65 Arsenic (nph) How many measurements taken in this time frame were within [45, 50) ppb? ans =