An observation from a normally distributed population is considered "unusual" if it is more than 2 standard deviations away from the mean. There are several contaminants that can harm a city's water supply. Nitrate concentrations above 10 ppm (parts per million) are considered a health risk for infants less than six month of age. The City of Rexburg reports that the nitrate concentration in the city's drinking water supply is between 1.59 and 2.52 ppm (parts per million,) and values outside of this range are unusual. We will assume LaTeX: \mu-2\sigma=1.59μ − 2 σ = 1.59 ppm and LaTeX: \mu+2\sigma=2.52μ + 2 σ = 2.52 ppm. It is reasonable to assume the measured nitrate concentration is normally distributed. (Source: City of Rexburg)