The EPA wants to test a randomly selected sample of $n$ water specimens and estimate the mean daily rate of pollution produced by a mining operation. If the EPA wants a $95$% confidence interval with a bound of error of $0.5$ milligram per liter (mg/L), how many water specimens are required in the sample? Assume prior knowledge indicates that pollution readings in water samples taken during a day have been approximately normally distributed with a standard deviation of $3.5$ (mg/L).

$n =$