Do you want this in code or as a formula?
Try
=IF(A1+B1<=120,"",B1)
or
=IF(A1+B1<=120,A1+B1,B1)
or
=IF(A1+B1<=120,A1+B1,IF(B1<=20,B1,""))

"John Savy" wrote:

Could you write a formula to function as:
"A1 + B1 = 120"
- assuming B1 consists of numbers from 1 to 20, that means the formula
should calculate the value of A1 plus B1 (1, 2, ..20).
If the the summation becomes equal 120, then the sub-number of B1 (which is
any number between 1 to 20) otherwise blank.
--
John