\[\boxed{\mathbf{2}\mathbf{.}}\]
\[21 - 3 = 21 - 1 - 2 = 20 - 2 =\]
\[= 18\]
\[22 - 5 = 22 - 2 - 3 = 20 - 3 =\]
\[= 17\]
\[31 - 2 = 31 - 1 - 1 = 30 - 1 =\]
\[= 29\]
\[32 - 4 = 32 - 2 - 2 = 30 - 2 =\]
\[= 28\]
\[41 - 5 = 41 - 1 - 4 = 40 - 4 =\]
\[= 36\]
\[52 - 3 = 52 - 2 - 1 = 50 - 1 =\]
\[= 49\]