Order of Element in Quotient Group

Theorem
Let $G$ be a group, and let $H$ be a normal subgroup of $G$.

Let $G / H$ be the quotient group of $G$ by $H$.

The order of $a H \in G / H$ divides the order of $a \in G$.

Proof
Let $G$ be a group with normal subgroup $H$.

Let $G / H$ be the quotient of $G$ by $H$.

From Natural Group Epimorphism is Epimorphism, $G/H$ is a homomorphic image of $G$.

Let $f: G \to G/H$ given by $f \left({a}\right) = aH$ be that homomorphism.

Let $a \in G$ such that $a^n = e$ for some integer $n$.

Then, by the morphism property of $f$:

Hence $\left|{H}\right|$ divides $n$.