Let $f : X \to Y$ be a D18: Map such that

(i)	$B_n \subseteq Y$ is a D78: Subset of $Y$ for each $n \in \mathbb{N}$

Then \begin{equation} f^{-1} \left( \bigcup_{n \in \mathbb{N}} B_n \right) = \bigcup_{n \in \mathbb{N}} f^{-1}(B_n) \end{equation}