\[\boxed{\mathbf{215}.}\]
\[C = \left\{ x:\ \ x\ \vdots 18 \right\};\]
\[\ \ D = \left\{ y:\ \ y \vdots 45 \right\}:\]
\[C = \left\{ 18;36;54;\ldots \right\};\ \]
\[\ \ D = \left\{ 45;90;135;\ldots \right\}\]
\[C \cap D = \left\{ 90;180;\ldots \right\}.\]
\[НОК(18;\ 45) = 90.\]