Preliminary analysis of a few soil samples from a parking lot with historical lead-gasoline spill and lead paint from the demolished buildings showed soil Pb concentrations between 60 and 700 mg/kg, and a standard deviation of 20.9 mg/kg. The number of samples required is _____ so that the estimated sample mean would be within ± 15 mg/kg of the population mean at 95% confidence level (Use trial-and-error to solve samplen).