For example you have number A and B.  A+B = C  A-B = D  (A+B)+(A-B)=2A  Thus, C+D must always be even number. Cannot be odd number. 
Is it possible to find two numbers such that if you add up the sum and the difference of these two numbers, you get $$999$$? If yes, please write it out. If not, please give your reason. 

a. Possible 
b. Not possible 