Essentially, the rule goes as so: odd + odd = even, even + even = even.
Although that may not be the answer you're looking for, it is a math rule that is never debunked and essentially the premises for why you get an even sum practically always depending on the numbers.
Hope this helps!
A: adding together 2 and 3 a bunch of times
2+3+2+3+2+3 etc does not equal 28. a possible answer can be 27 students.