Cardinality_of_Complement

Let $T \subseteq S$ such that $\card S = n, \card T = m$. Then $\card {\relcomp S T} = \card {S \setminus T} = n - m$ where $\relcomp S T$ denotes the complement of $T$ relative to $S$ $S \setminus T$ denotes the difference between $S$ and $T$.