\[\boxed{\mathbf{17.}}\]
\[\textbf{а)}\ 39 - 10 = 29\]
\[39 - 20 = 19\]
\[39 - 30 = 9\]
\[53 - 1 = 52\]
\[53 - 3 = 50\]
\[53 - 30 = 23\]
\[76 - 6 = 70\]
\[76 - 60 = 16\]
\[76 - 70 = 6\]
\[\textbf{б)}\ 48 - 5 - 30 = 43 - 30 = 13\]
\[65 - (13 - 8) = 65 - 5 = 60\]
\[87 - (20 - 13) = 87 - 7 = 80\]