\[\boxed{\mathbf{21.}}\]
\[\textbf{а)}\ 30 - 3 = 27\]
\[50 - 4 = 46\]
\[100 - 2 = 98\]
\[60 - 15 = 45\]
\[40 - 12 = 28\]
\[50 - 23 = 27\]
\[\textbf{б)}\ 80 - 8 + 20 = 72 + 20 = 92\]
\[90 - 11 - 4 = 79 - 4 = 75\]
\[100 - 7 - 31 = 93 - 31 = 62\]
\[52 + (17 - 9) = 52 + 8 = 60\]
\[60 - (6 + 7) = 60 - 13 = 47\]
\[100 - (14 - 8) = 100 - 6 = 94\]