\[\boxed{\mathbf{29.}}\]
\[\textbf{а)}\ 10 + 6\ :2 = 10 + 3 = 13\]
\[11 + 8\ :2 = 11 + 4 = 15\]
\[1 + 20\ :2 = 1 + 10 = 11\]
\[15 - 6\ :3 = 15 - 2 = 13\]
\[17 - 8\ :4 = 17 - 2 = 15\]
\[20 - 10\ :2 = 20 - 5 = 15\]
\[\textbf{б)}\ 9 + 12\ :2 = 9 + 6 = 15\]
\[11 - 15\ :3 = 11 - 5 = 6\]
\[16\ :4 + 7 = 4 + 7 = 11\]