TITLE: What is the quotient of a cyclic group of order $n$ by a cyclic subgroup of order $m$?
QUESTION [7 upvotes]: Suppose that $H$ is a (cyclic) subgroup of order $m$ of a cyclic group
  $G$ of order $n$. What is $G/H$?

It's a very simple question but I am still struggling with getting accustomed to the notion of the quotient group. 
Now, I know that $m$ divides $n$ and that $G/H$ will be a group of cosets such that $\{gH:g\in G\}$. As a result, there will be $n/m$ of such cosets. I was wondering I can say anything more specific than just that?

REPLY [7 votes]: Yuval's answer is of course correct, but let's add a bit more detail (I assume you are still a bit new to groups).
Since $G$ is cyclic, by definition $G = \langle x \rangle$ for some $x \in G$.Let's PROVE that for any subgroup $H$ of $G$, $xH$ generates $G/H$.
Note that any element of $G/H$ is of the form $gH = \{gh: h \in H\}$ for some element $g \in G$. Since $G = \langle x \rangle$, $g = x^k$ for some integer $k$. Thus $gH = (x^k)H = (xH)^k$, from the definition of coset multiplication. This shows $xH$ generates $G/H$.
Let's look at an example, to see how this really works:
Suppose $G = \Bbb Z_6 = \{0,1,2,3,4,5\}$ under addition modulo 6, and that $H = \{0,2,4\}$. We can list the cosets explicitly (there's only two of them):
$H = 2+H = 4+H = \{0,2,4\}$
$1+H = 3+H = 5+H = \{1,3,5\}$.
All of the possible products are given by:
$H + H = H$
$H + (1+H) = (1+H) + H = 1+H$
$(1+H) + (1+H) = 2 + H = H$
Thus $G/H = \{H,1+H\}$ and is generated by $1+H$ (because $G$ is generated by $1$), which is an element of order 2 (and $G/H$, being cyclic, is of the same order (as a group) as the order of its generator (as an element)).