What is an odd number minus odd number?

Answer 1

All odd numbers are of the form 2n + 1, where n is an integer.

So an odd number minus an odd number is (2n+1) - (2m+1) = 2n -2m = 2(n-m). Both n and m are integers, so while we don't know whether n-m is odd or even, we definitely know that it's an integer and that multiplying it by two cannot possibly give an odd number. So an odd number minus an odd number is an even number. For similar reasons, an odd number plus an odd number is also an even number.