The question is
If $x$ and $y$ are prime numbers , which of the following can not be their sum? $5$,$9$,$13$,$16$ or $23$.
The answer is $23$.
How did they get this? As far as I can tell is that when prime numbers are added I am suppose to get an even value for example $5+7 = 12 $ or $7+7 = 14 $ or $7+11 = 18 $. Could anyone please tell me what I am missing here?