§ Задание 12
\[\boxed{\mathbf{12}\mathbf{.}}\]

\[\textbf{б)}\ 21 - 6 + 15 = 15 + 15 = 30\]
\[17 + 14 - 2 = 31 - 2 = 29\]
\[4 + 51 - 8 = 55 - 8 = 47\]
\[6 + 36 - 7 = 42 - 7 = 35\]
\[\boxed{\mathbf{12}\mathbf{.}}\]

\[\textbf{б)}\ 21 - 6 + 15 = 15 + 15 = 30\]
\[17 + 14 - 2 = 31 - 2 = 29\]
\[4 + 51 - 8 = 55 - 8 = 47\]
\[6 + 36 - 7 = 42 - 7 = 35\]