  Let $R$ be a ring with identity, $\mathcal{U}(R)$ the group of units of $R$
and $k$ a positive integer. We say that $a\in \mathcal{U}(R)$ is $k$-unit if
$a^k=1$. Particularly, if the ring $R$ is $\mathbb{Z}_n$, for a positive
integer $n$, we will say that $a$ is a $k$-unit modulo $n$. We denote with
$\mathcal{U}_k(n)$ the set of $k$-units modulo $n$. By $\text{du}_k(n)$ we
represent the number of $k$-units modulo $n$ and with
$\text{rdu}_k(n)=\frac{\phi(n)}{\text{du}_k(n)}$ the ratio of $k$-units modulo
$n$, where $\phi$ is the Euler phi function. Recently, S. K. Chebolu proved
that the solutions of the equation $\text{rdu}_2(n)=1$ are the divisors of
$24$. The main result of this work, is that for a given $k$, we find the
positive integers $n$ such that $\text{rdu}_k(n)=1$. Finally, we give some
connections of this equation with Carmichael's numbers and two of its
generalizations: Kn\"odel numbers and generalized Carmichael numbers.
