The sum of two odd numbers is odd

Explanation

Take two arbitrary odd numbers, written as 2a + 1 and 2b + 1 for integers a and b. Their sum is (2a + 1) + (2b + 1) = 2a + 2b + 2 = 2(a + b + 1). Since a + b + 1 is an integer, the result is a multiple of 2, which is exactly the definition of an even number. For example, 3 + 7 = 10.