Length of Subgroup Plus Length of Quotient Group

Theorem
Let $G$ be a finite group.

Let $H$ be a normal subgroup of $G$.

Then:
 * $l \left({G}\right) = l \left({H}\right) + l \left({G / H}\right)$

where:
 * $l \left({G}\right)$ denotes the length of $G$
 * $G / H$ denotes the quotient group of $G$ by $H$.