Expected mean and standard deviation

[Guest]

2. The salaries of a group of 10000 engineers are normally distributed with a mean of 4000TL and a standard deviation of 500TL. We will randomly choose 100 engineers from this population, phone them up and ask what their salary is. We then use this data to calculate the average salary for the engineers. Determine the expected mean and standard deviation of the average salary that we will calculate if
a. When we phone an engineer, we cross his name off the list so we do not call him again.
b. When we phone an engineer, we don't cross his name off the list, we may phone him again and use his salary more than once in our calculations.