\[\boxed{\mathbf{11.}}\]
\[30 - 12 = 30 - 10 - 2 =\]
\[= 20 - 2 = 18\]
\[40 - 15 = 40 - 10 - 5 =\]
\[= 30 - 5 = 25\]
\[50 - 16 = 50 - 10 - 6 =\]
\[= 40 - 6 = 34\]
\[50 - 23 = 50 - 20 - 3 =\]
\[= 30 - 3 = 27\]
\[60 - 25 = 60 - 20 - 5 =\]
\[= 40 - 5 = 35\]
\[70 - 24 = 70 - 20 - 4 =\]
\[= 50 - 4 = 46\]
\[60 - 47 = 60 - 40 - 7 =\]
\[= 20 - 7 = 13\]
\[80 - 32 = 80 - 30 - 2 =\]
\[= 50 - 2 = 48\]
\[90 - 51 = 90 - 50 - 1 =\]
\[= 40 - 1 = 39\]