At the 95% confidence level, the confidence interval is given by (21 - 1.96(standard deviation)/(square root of number of samples), 21 + 1.96(standard deviation)/(square root of number of samples)). Here, we do not know the standard deviation and the sample size. Since the number 1.96(standard deviation)(square root of number of samples) is probably small, the only likely number to be outside the confidence interval is 15.
Therefore, we can conclude that 15 is probably not the mean number of days that the company's air fresheners last.
