The product of two whole numbers is $$42$$. Their sum \emph{cannot} be． 
Overseas Competition->Knowledge Point->Number Theory Modules->Factors and Multiples->Basic Concepts of Factors and Multiples
All but $$33$$ can be represented as required, as shown below.  A. $$43=1+42$$  B. $$33$$  C. $$23=2+21$$  D. $$13=6+7$$ 