\[\boxed{\mathbf{2}\mathbf{.}}\]
\[31 - 12 = 31 - 10 - 2 =\]
\[= 21 - 2 = 19\]
\[32 - 15 = 32 - 10 - 5 =\]
\[= 22 - 5 = 17\]
\[42 - 13 = 42 - 10 - 3 =\]
\[= 32 - 3 = 29\]
\[41 - 23 = 41 - 20 - 3 =\]
\[= 21 - 3 = 18\]
\[43 - 24 = 43 - 20 - 4 =\]
\[= 23 - 4 = 19\]
\[52 - 25 = 52 - 20 - 5 =\]
\[= 32 - 5 = 27\]
\[62 - 13 = 62 - 10 - 3 =\]
\[= 52 - 3 = 49\]
\[63 - 25 = 63 - 20 - 5 =\]
\[= 43 - 5 = 38\]
\[71 - 36 = 71 - 30 - 6 =\]
\[= 41 - 6 = 35\]