I want to prove this.
Let $f$ is a bijection between $A$ and $B$,
$A$ is a countably infinite set if and only if $B$ is a countably infinte set.
I use definition A is a countably infinite set then there exist a bijection between A and $\mathbb N$,
but i don't know to show that $B$ is a countably infinite set.
Please, give me a hint or prove this.