Solution
= Solution
Let $j:B\to\mathbb N$ be an <injection> witnessing that $B$ is a <countable set>. The composition $j\circ f:A\to\mathbb N$ is an <injection>, so $A$ is a <countable set>. This proof includes finite and empty sets.
= Solution
Let $j:B\to\mathbb N$ be an <injection> witnessing that $B$ is a <countable set>. The composition $j\circ f:A\to\mathbb N$ is an <injection>, so $A$ is a <countable set>. This proof includes finite and empty sets.