Sorry Joanne for making you confused.
Write down the function where you are adding or substracting the
numbers as :
=if(E1="YES",A1+B1,if(E1="NO",A1-B1,""))
A1 and B1 are just examples to tell you how you write a function
And at the column you added just now for telling to Add or Substracted
do as below:
1. Select Data / Validation
2. Choose Allow - Select List
3. Write Yes, No ie., Yes (comma) No
Am I making it clear to you.
You can send me an email with the XLS
Thanks
Shail