The standard deviation of a particular sample is equal to the standard deviation divided by the square root of the sample size. That is,[tex]\sigma_x=\frac{\sigma}{\sqrt n}[/tex]
The mean of a particular sample is equal to the mean of the set the sample was taken from. That is,
[tex]\mu_x=\mu[/tex]
[tex]\text{In your case, }\sigma_x=\frac{9}{\sqrt4}=\frac92\text{ and }\mu_x=45.[/tex]
