\[\boxed{\mathbf{6.}}\]
\[\textbf{а)}\ 37 + 3 - 30 = 40 - 30 = 10\]
\[61 - 2 + 20 = 59 + 20 = 79\]
\[6 + 24 - 8 = 30 - 8 = 22\]
\[100 - 40 - 4 = 60 - 4 = 56\]
\[\textbf{б)}\ 43 + (61 - 4) = 43 + 57 =\]
\[= 100\]
\[77 - 7 - 68 = 70 - 68 = 2\]