Combine Data Into One Row, Add Columns

Hey guys,

I realize I've posted something like this before, but i'm confused how to do something new, what I am trying to do is combine rows of data when certain columns equal one another.

For example, I have to sets of data who have in common the columns ctp_code and a mod_code (they are the same) however they have two other columns called billing_amt and amount_owed that need to be added together (literally taking the data and doing the math, so 100.00 + 100.00 = 200.00)


Code:

id ctp_code mod_code billing_amt amount_owed
1 1 1 100.00 100.00
2 1 1 100.00 100.00



My results should be this:


Code:

id ctp_code mod_code billing_amt amount_owed
1 1 1 200.00 200.00



Any help, thoughts?