Number is Divisor iff Modulo is Zero

Theorem
Let $x, y \in \R$ be real numbers.

Let $x \bmod y$ denote the modulo operation:
 * $x \bmod y := \begin {cases}

x - y \floor {\dfrac x y} & : y \ne 0 \\ x & : y = 0 \end {cases}$ where $\floor {\dfrac x y}$ denotes the floor of $\dfrac x y$.

Then $x \bmod y = 0$ $x$ is an integer multiple of $y$.

Sufficient Condition
Let $x \bmod y = 0$.

From Number minus Modulo is Integer Multiple:
 * $x - \paren {x \bmod y}$

is an integer multiple of $y$.

As $x \bmod y = 0$ it follows that $x - 0 = x$ is an integer multiple of $y$.

Necessary Condition
If $y = 0$ it follows from Zero Divides Zero that $x = 0$.

Hence the result by definition of the modulo operation:

Otherwise, let $y \ne 0$.

Let $x$ be an integer multiple of $y$.

That is:
 * $\exists n \in \Z: x = n y$

Hence:
 * $\dfrac x y = n$

From Quotient of Modulo Operation with Modulus:
 * $\dfrac x y - \floor {\dfrac x y} = \dfrac {x \bmod y} y$

From Real Number is Integer iff equals Floor it follows that:
 * $\dfrac x y = \floor {\dfrac x y}$

Thus:
 * $\dfrac {x \bmod y} y = 0$

and so multiplying both sides by $y$ the result follows.