\[\boxed{\mathbf{19}\mathbf{.}}\]
\[\textbf{б)}\ 2\ см \cdot 9 = 18\ см\]
\[12\ см\ :3 = 4\ см\]
\[16\ см\ :2 = 8\ см\]
\[15\ м\ :3 \cdot 2 = 5\ м \cdot 2 = 10\ м\]
\[24\ м\ :3 \cdot 2 = 8\ м \cdot 2 = 16\ м\]
\[16\ м\ :4 \cdot 3 = 4\ м \cdot 3 = 12\ м\]