\[\boxed{\mathbf{15.}}\]
\[\textbf{а)}\ 40 + 8 = 48\]
\[60 + 5 = 65\]
\[90 - 1 = 89\]
\[60 + 10 = 70\]
\[50 - 10 = 40\]
\[90 + 10 = 100\]
\[\textbf{б)}\ 70 + 10 + 10 = 80 + 10 = 90\]
\[80 - 10 - 10 = 70 - 10 = 60\]
\[100 - 10 - 10 = 90 - 10 = 80\]