Order of Subgroup Product

Theorem
Let $G$ be a group.

Let $H$ and $K$ be subgroups of $G$.

Then:
 * $\order {H K} = \dfrac {\order H \order K} {\order {H \cap K} }$

where:
 * $H K$ denotes subset product
 * $\order H$ denotes the order of $H$.