Use the sample size formula:
n >= ((Za)^2) * (s/d)^2
plug in 1.96 for Za (corresponding to 95%), plug in 4 for s, and 0.4 for d
A random sample of size n is taken from a Normal Distribution having a standard deviation od 4. Find the least value of n for which the probability that the sample mean lies within 0.4 of the population mean exceeds 0.95.
Any help greatly appreciated.