\[\boxed{\mathbf{6.}}\]
\[\textbf{а)}\ 30 + 10 = 40\]
\[60 + 10 = 70\]
\[70 + 10 = 80\]
\[40 - 10 = 30\]
\[60 - 10 = 50\]
\[80 - 10 = 70\]
\[\textbf{б)}\ 20 + 10 + 10 = 30 + 10 = 40\]
\[50 + 10 + 10 = 60 + 10 = 70\]
\[50 - 10 - 10 = 40 - 10 = 30\]