A small-business owner must hire seasonal workers as the need arises. The following list shows the number of employees hired monthly for a 5-month period.

4, 13, 5, 6, 9

If the mean of these data is approximately 7, what is the population standard deviation for these data?

I have never done standard deviation in my class before-this question was from a sample of the CAHSEE. Please help and explain how I can do this problem.