So, if your data is what I think it is, you could select the records for the single type you want to run the calculation against, from the open table right-click on the area field and get the summary stats (this dynamic tool is denoted by the sigma sign, I believe), so that (if you don't already have the sum figure) you will then have a sum of the area of those selected records.
You could copy this figure, create a new numeric field and (conceivably) calculate the division by 'hard-coding' this value in the field calculator expression. Of course it is not necessary to calculate the records from which you got the sum figure, so you could 'switch selection' before you run the calc. Make sense?
It may help if you attach a more representative table of your data...
Hope that helps.
Wayne