at is the minimum number of samples that need to be averaged in order to be 95% confident that the average of the means is writhin 2 of the true mean?...

If a population has a standard deviation of 18 what is the minimum number of samples that need to be averaged in order to be 95% confident that the average of the means is writhin 2 of the true mean? possible answers: 612; 18; 312; 36Please show me how to work this so I can "get it".Thanks.