Though the fields names will be different, you can follow along using my table calculations training video found here: http://www.tableausoftware.com/learn/tutorials/on-demand/table-calculations-0
You can skip to 10 minutes to see an example of what you want.
Hope this helps,
Thanks for the link Dan! The video show what I was looking to accomplish.
By the way thank you very much for posting these training videos, I got all my Tableu 7 education by watching them and it has been a great source of information. Great Job.