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. O $ 10 15 20 35 30 35 40 45 50 53 Arsenic (epb) Q How many measurements taken in this time frame were within [40, 45) ppb? ans =