\[\boxed{\mathbf{5.}}\]
\[\textbf{а)}\ 43 + 21 = 64\]
\[64 + 3 = 67\]
\[55 + 5 = 60\]
\[52 - 2 = 50\]
\[37 - 30 = 7\]
\[30 - 6 = 24\]
\[\textbf{б)}\ 56 + 30 - 2 = 86 - 2 = 84\]
\[73 - 10 + 24 = 63 + 24 = 87\]
\[80 - 2 - 70 = 78 - 70 = 8\]
\[40 - (14 - 5) = 40 - 9 = 31\]
\[56 + (7 + 7) = 56 + 14 = 70\]
\[100 - (13 - 8) = 100 - 5 = 95\]