How do I do this in Excel with a formula or function?

I have this spreadsheet:
http://img.photobucket.com/albums/v214/wandarer/spreadsheet.jpg

In it, I need to make it show that I18 (M18 etc) all show the total number of trips during that bill (So I18 should show 4), but I have to do it using a function or formula that can be used, without being changed, in the other cells (so that the formula in I18 can be copied/pasted into M18 and still work)

How would I do that?
I typed the original explanation wrong (it's late ;) ) along with needing to sum the different categories row 18, I also need to be able to add up how many "trips" were taken for each bill.. i.e. for "Bill #1", I have to find a way to calculate that there were 4 trips in that trip (to be displayed in I18), but I also need to be able to use the same function to calculate the number of trips for the rest of the bills - i.e. to show that for "Bill #2" there were 3 trips (to be displayed in I21)