\[\boxed{\mathbf{1.}}\]
\[A = \left\{ 0;2;4;6;8 \right\}:\]
\[4 \in A\]
\[5 \notin A\]
\[8 \in A\]
\[36 \notin A\]
\[\boxed{\mathbf{2.}}\]
\[\boxed{\mathbf{3.}}\]
\[28\ :9 = 3\ (ост.\ 1)\]
\[Проверка:\]
\[3 \cdot 9 + 1 = 27 + 1 = 28.\]
\[75\ :6 = 12\ (ост.\ 3)\]
\[Проверка:\]
\[12 \cdot 6 + 3 = 72 + 3 = 75.\]
\[\boxed{\mathbf{4.}}\]
\[(70 - 20)\ :2 = 50\ :2 = 25\ (л) -\]
\[бензина\ в\ каждой\ канистре.\]
\[Ответ:25\ л.\]
\[\boxed{\mathbf{5.}}\]
\[2 \in C\]
\[2 \notin D\]
\[b \in C\]
\[b \in D\]
\[\mathrm{\Delta}\ \notin C\]
\[\mathrm{\Delta}\ \in D\]
\[\odot \ \notin C\]
\[\odot \ \notin D\]