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)

Given that 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.

We can say 1.59 and 2.52 if taken average would be the mean

Mean = 2.01

The deviations on either side = 2.52-2.01 = 0.41

This can be taken as 2 std dev

So std dev = 0.205

Hence we can take nitrate concentration is normal with mean = 2.01 and std deviation is 0.205

N(2.01, 0.205)