\[\boxed{\mathbf{5.}}\]
\[\textbf{а)}\ 40 + 10 = 50\]
\[70 + 10 = 80\]
\[90 + 10 = 100\]
\[30 - 10 = 20\]
\[60 - 10 = 50\]
\[100 - 10 = 90\]
\[\textbf{б)}\ 60 + 10 + 10 = 70 + 10 = 80\]
\[50 - 10 - 10 = 40 - 10 = 30\]
\[10 + 90 - 10 = 100 - 10 = 90\]
\[70 - 10 - 10 = 60 - 10 = 50\]
\[80 + 10 + 10 = 90 + 10 = 100\]
\[90 - 10 - 10 = 80 - 10 = 70\]