\[\boxed{\mathbf{19.}}\]
\[\textbf{а)}\ 23 + 13 = 36\]
\[16 + 42 = 58\]
\[35 + 61 = 86\]
\[67 - 14 = 53\]
\[86 - 34 = 52\]
\[95 - 23 = 72\]
\[\textbf{б)}\ 24 + (9 + 3) = 24 + 12 = 36\]
\[(12 - 5) + 51 = 7 + 51 = 58\]
\[66 - (8 + 3) = 66 - 11 = 55\]
\[(19 - 12) + 50 = 7 + 50 = 57\]
\[60 + (34 - 4) = 60 + 30 = 90\]
\[100 - (3 + 17) = 100 - 20 = 80\]