\[\boxed{\mathbf{17}\mathbf{.}}\]
\[\textbf{а)}\ 100 - 2 = 98\]
\[100 - 12 = 88\]
\[100 - 22 = 78\]
\[\textbf{б)}\ 50 - (16 + 7) = 50 - 23 = 27\]
\[40 - (24 + 9) = 40 - 33 = 7\]
\[60 - (3 + 38) = 60 - 41 = 19\]
\[4 + 67 - 50 = 71 - 50 = 21\]
\[8 + 89 - 5 = 97 - 5 = 92\]
\[6 + 94 - 0 = 100 - 0 = 100\]