excel function reduce and increase numbers

[Guest]

I need to find out a formula that increases one number when the other
decreases, but does not decrease the number when the other increases.
example
cell A1=30 B1=40
NEXT
cell A1=31 B1=39
but not when there is an increase
cell A1=31 B1=42

 

[Barb Reinhardt]

I'm thinking you'll need to have a helper column to do this or else you'll
have circular references.