A Note on Finite Nilpotent Groups

It is well known that if $G$ is a group and $H$ is a normal subgroup of $G$ of finite index $k$, then $x^k \in H$ for every $x \in G$. We examine finite groups $G$ with the property that $x^k \in H$ for every subgroup $H$ of $G$, where $k$ is the index of $H$ in $G$. We prove that a finite group $G$ satisfies this property if and only if $G$ is nilpotent.