Help!! Seemed to be stuck on this problem, can anyone help?? Thanks!

In order to help identify baby growth pattern that are unusual we need to construct a confidence interal estimate of the mean head circumference of all babies that are two months old. A random sample of 100 babies is obtained the mean head circumference is found to be 40.6cm. Assuming that the population is known to be 1.6cm, find 99% confidence interval estimate of the mean head circumference of all two month old babies. What aspect of the problem is not realistic?

confidence level 99% critical value of 99% is =2.575

sinlee