\[\boxed{\mathbf{63}\mathbf{.}}\]
\[30 - 13 = 30 - 10 - 3 =\]
\[= 20 - 3 = 17\]
\[40 - 11 = 40 - 10 - 1 =\]
\[= 30 - 1 = 29\]
\[50 - 21 = 50 - 20 - 1 =\]
\[= 30 - 1 = 29\]
\[60 - 24 = 60 - 20 - 4 =\]
\[= 40 - 4 = 36\]
\[70 - 42 = 70 - 40 - 2 =\]
\[= 30 - 2 = 28\]
\[80 - 36 = 80 - 30 - 6 =\]
\[= 50 - 6 = 44\]
\[70 - 37 = 70 - 30 - 7 =\]
\[= 40 - 7 = 33\]
\[80 - 51 = 80 - 50 - 1 =\]
\[= 30 - 1 = 29\]
\[90 - 42 = 90 - 40 - 2 =\]
\[= 50 - 2 = 48\]