Twenty air samples taken at the same site over a period of 6 months showed the following amounts of suspended particulate matter (micrograms per cubic meter of air):

68 22 36 32 42 24 28 38 30 44 28 27 28 43 45 50 79 74 57 21 Consider these measurements to be a random sample from a population of normally distributed measurements, and construct a 95 percent confidence interval for the population variance.