\[\boxed{\mathbf{11.}}\]
\[\textbf{а)}\ 38 - 15 = 23\]
\[55 - 23 = 32\]
\[67 - 42 = 25\]
\[49 - 35 = 14\]
\[75 - 14 = 61\]
\[96 - 21 = 75\]
\[\textbf{б)}\ 58 - 17 - 1 = 41 - 1 = 40\]
\[74 - 52 - 1 = 22 - 1 = 21\]
\[86 - 21 - 1 = 65 - 1 = 64\]
\[47 - 26 - 10 = 21 - 10 = 11\]
\[63 - 31 - 10 = 32 - 10 = 22\]
\[98 - 45 - 10 = 53 - 10 = 43\]