Copying Formulea from one cell to another

I want to drag and paste a formulea across a row. But the formulea takes
information from another cell, then will miss two columns out, and themn
takes information from the next, and so on. I can't seem to find a way of
copying the pattern, and I am wasting a lot of time editing each formulea
manually.


i.e 1st cell should read '=SUM(L15:L10)' 2nd cell next to it should read
'=SUM(O15:O10)', the next '=SUM(R15:R10)', and so on.

I would like to know a way of doing this automatically.

I am using Excel 2003 Standard Edition

And would be grateful for advice.
--
Mark