\[\boxed{\mathbf{16}\mathbf{.}}\]
\[\textbf{а)}\ 18 + 3 + 3 = 18 + 6 = 24\]
\[24 + 3 + 3 = 24 + 6 = 30\]
\[21 - 3 - 3 = 18 - 3 = 15\]
\[30 - 3 - 3 = 27 - 3 = 24\]
\[\textbf{б)}\ 61 - (8 + 6) = 61 - 14 = 47\]
\[78 + (31 - 9) = 78 + 22 = 100\]