how many measurements taken in this frame were within [40,45)ppb?

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. 10 5 10 5 20 25 40 35 40 45 50 55 How many measurements taken in this time frame were within [40, 45) ppb? ans =