Let $X$ be a D17: Finite set such that

(i) | $E \subseteq X$ is a D78: Subset of $X$ |

Then
\begin{equation}
|E|
= |X| - |X \setminus E|
\end{equation}

Set cardinality in terms of finite ambient set

Formulation 0

Let $X$ be a D17: Finite set such that

(i) | $E \subseteq X$ is a D78: Subset of $X$ |

Then
\begin{equation}
|E|
= |X| - |X \setminus E|
\end{equation}

Proofs

Proof 0

Let $X$ be a D17: Finite set such that

(i) | $E \subseteq X$ is a D78: Subset of $X$ |

This result is a particular case of R4189. $\square$