\[\boxed{\mathbf{20.}}\]
\[\textbf{а)}\ 51 + 16 = 67\]
\[35 - 12 = 23\]
\[43 + 5 = 48\]
\[75 - 20 = 55\]
\[62 - 30 = 32\]
\[2 + 45 = 47\]
\[37 - 27 = 10\]
\[48 - 4 = 44\]
\[\textbf{б)}\ 74 - 10 + 25 = 64 + 25 = 89\]
\[53 + 3 - 14 = 56 - 14 = 42\]
\[92 - 40 + 31 = 52 + 31 = 83\]
\[1 + 69 - 50 = 70 - 50 = 20\]
\[30 + (16 - 8) = 30 + 8 = 38\]
\[(80 - 30) + 46 = 50 + 46 = 96\]
\[67 - (9 + 6) = 67 - 15 = 52\]
\[100 - (55 - 5) = 100 - 50 = 50\]