§ Задание 21
\[\boxed{\mathbf{21.}}\]
\[2 \cdot 6 = 12\]
\[12\ :2 = 6\]
\[12\ :6 = 2\]
\[3 \cdot 5 = 15\]
\[15\ :3 = 5\]
\[15\ :5 = 3\]
\[5 \cdot 4 = 20\]
\[20\ :4 = 5\]
\[20\ :5 = 4\]
\[\boxed{\mathbf{21.}}\]
\[2 \cdot 6 = 12\]
\[12\ :2 = 6\]
\[12\ :6 = 2\]
\[3 \cdot 5 = 15\]
\[15\ :3 = 5\]
\[15\ :5 = 3\]
\[5 \cdot 4 = 20\]
\[20\ :4 = 5\]
\[20\ :5 = 4\]