\[\boxed{\mathbf{29}\mathbf{.}}\]
\[\textbf{а)}\ 26 - 6 = 20\]
\[37 - 7 = 30\]
\[55 - 5 = 50\]
\[64 - 4 = 60\]
\[95 - 90 = 5\]
\[88 - 80 = 8\]
\[\textbf{б)}\ 36 - (17 - 11) = 36 - 6 =\]
\[= 30\]
\[42 - (20 - 18) = 42 - 2 = 40\]
\[93 - (19 - 16) = 93 - 3 = 90\]