\[\boxed{\text{455\ (455).}\text{\ }\text{Еуроки\ -\ ДЗ\ без\ мороки}}\]
Пояснение.
Решение.
\[1)\ \left\{ a,\ b \right\}\ \cup \ \left\{ b \right\} = \left\{ a,\ b \right\};\]
\[2)\ \left\{ a,b \right\}\ \cup \ \left\{ b \right\} \neq \left\{ b \right\};\]
\[3)\ \left\{ a,b \right\}\ \cup \ \left\{ b \right\} \neq \left\{ a \right\};\]
\[4)\ \left\{ a,b \right\}\ \cup \ \left\{ b \right\} \neq \left\{ \left\{ b \right\} \right\}\text{.\ }\]