You can put this solution on YOUR website!
No matter what value
is one more than
is one less than
They are clearly two different numbers and
cannot be two different numbers at the same time.
A different explanation would involve functions, graphs, geometry, triangles, and proportions. All of that is beautifully intuitive math that does not require algebra, but it is not customary to talk about that to people who have not learned algebra. (I see no good reason to introduce math concepts in one order or another, but it is hard to fight tradition).