\[\boxed{\mathbf{13.}}\]
\[\textbf{а)}\ 43 + 23 = 66\]
\[38 + 41 = 79\]
\[57 - 15 = 42\]
\[95 - 13 = 82\]
\[\textbf{б)}\ 23 + (9 + 5) = 23 + 14 = 37\]
\[55 - (8 + 3) = 55 - 11 = 44\]
\[(18 - 12) + 50 = 6 + 50 = 56\]
\[100 - (4 + 16) = 100 - 20 = 80\]