Show that a group of order $5$ is always abelian.
I know that if the binary operation $*$ is defined on a group $G$ is commutative (i.e. $a*b=b*a\ \forall a,b\in G$),\ then G is called a commutative(or abelian) group. But I don't know how to solve a group of order $5$ is abelian. Thanks in advance!