Didn't know that one forum member could move another member's post around like this.
Here's the general calculation, but whether the fields are or are not defined in the same table can affect both the syntax needed and the results produced by it.
Field C can be defined to have this calculation:
If ( Job type = "Business" ; A + B ; Job type = "Domestic" ; A )
If you prefer, you can replace the word "If" with "Case" and get the same result. (You can look up "If" and "Case" functions in FileMaker Help to learn their differences if interested.)