\[\boxed{\mathbf{46.}}\]
\[\textbf{а)}\ 38 + 4\ :2 = 38 + 2 = 40\]
\[20 - 16\ :2 = 20 - 8 = 12\]
\[50 - 6\ :3 = 50 - 2 = 48\]
\[12\ :2 + 24 = 6 + 24 = 30\]
\[96 + 12\ :3 = 96 + 4 = 100\]
\[100 - 12\ :4 = 100 - 3 = 97\]
\[\textbf{б)}\ 80 - 18\ :2 = 80 - 9 = 71\]
\[20\ :5 + 86 = 4 + 86 = 90\]
\[100 - 16\ :4 = 100 - 4 = 96\]