"Christopher Brooks" wrote in message
...
How do I write a function, like =SUM(B6:B10) so that it
will operate on a range of cells selected in the
worksheet?


You can't. You could have event code to update a SUM cell on selection.

Also, is there a way to see the code for the built-in
functions so that I can reverse engineer?


Again, you can't, it is company confidential. If you want an algorithm, look
up on the web, there are plenty out there.