Water pollution testing. The EPA wants to test a randomly selected sample of n water specimens and estimate m, the mean daily rate of pollution produced by a mining operation.

If the EPA wants a 95% confidence interval estimate with a sampling error of 1 milligram per liter (mg/L), how many water specimens are required in the sample? Assume that prior knowledge indicates that pollution readings in water samples taken during a day are approximately normally distributed with a standard deviation equal to 5 (mg/L).