\[\boxed{\mathbf{16.}}\]
\[\textbf{а)}\ 17 + 36\ :6 = 17 + 6 = 23\]
\[21 - 18\ :6 = 21 - 3 = 18\]
\[42\ :6 + 25 = 7 + 25 = 32\]
\[\textbf{б)}\ 6 \cdot 7 - 15 = 42 - 15 = 27\]
\[61 - 5 \cdot 9 = 61 - 45 = 16\]
\[55 + 4 \cdot 7 = 55 + 28 = 83\]
\[100 - 30\ :6 = 100 - 5 = 95\]
\[35\ :5 + 93 = 7 + 93 = 100\]
\[9 - 54\ :6 = 9 - 9 = 0\]