finding sample size given info on confidence interval

1. The problem statement, all variables and given/known data



Suppose that an investigator believes that virtually all values in the population are between 38 and 70. The appropriate sample size for estimating the true population mean within 2 units with 95% confidence level is approximately how much?



2. Relevant equations





Variance of the sampling distribution of size n = Variance of the population distribution / n



3. The attempt at a solution





I think I'm solving

1.96(SD of the sampling distribution of size n)=2



But I don't think there's any information given on the population variance so I don't know what to do.