\[\boxed{\mathbf{21.}}\]
\[\textbf{а)}\ 40 - 2 = 38\]
\[54 + 6 = 60\]
\[30 - 13 = 17\]
\[20 + 65 = 85\]
\[70 - 6 = 64\]
\[17 + 43 = 60\]
\[\textbf{б)}\ 27 + 33 - 4 = 60 - 4 = 56\]
\[74 - 4 - 45 = 70 - 45 = 25\]
\[2 + 78 - 52 = 80 - 52 = 28\]
\[100 - 10 + 8 = 90 + 8 = 98\]
\[100 - 1 - 63 = 99 - 63 = 36\]
\[72 + 28 - 0 = 100 - 0 = 100\]