Without having discussed cyclic groups yet, in my textbook I find statements about isomorphisms like this:
$$ 2\mathbb{Z}/4\mathbb{Z} \cong \mathbb{Z}/2\mathbb{Z} $$
and
$$ S_n/A_n \cong \mathbb{Z}/2\mathbb{Z} $$
with $S_n$ being the symmetric group and $A_n$ the alternating group. I don't understand how to get to these statements. Can anyone help?