A random sample is selected from a normal population with a mean of = 100 and

Question:

A random sample is selected from a normal population with a mean of µ = 100 and a standard deviation of s = 20. After a treatment is administered to the individuals in the sample, the sample mean is found to be µ = 96.
a. How large a sample is necessary for this sample mean to be statistically significant? Assume a two-tailed test with a = .05.
b. If the sample mean were µ = 98, what sample size would be needed to be significant for a two tailed test with a = .05?