Question:
If $A, B, C$ are three sets such that $A \subset B$, then prove that $C-B \subset C-A$.
Solution:
Let $a \in C-B$
$\Rightarrow a \in C$ and $a \notin B$
$\Rightarrow a \in C$ and $a \notin A \quad[\because A \subset B]$
$\Rightarrow a \in C-A$
Hence, $C-B \subset C-A$