\[\boxed{\mathbf{19.}}\]
\[\textbf{а)}\ 42 + 1 = 43\]
\[67 - 1 = 66\]
\[1 + 55 = 56\]
\[60 - 10 = 50\]
\[51 + 10 = 61\]
\[28 - 10 = 18\]
\[\textbf{б)}\ 70 + 1 + 1 = 71 + 1 = 72\]
\[39 + 1 + 1 = 40 + 1 = 41\]
\[50 - 1 - 1 = 49 - 1 = 48\]
\[80 + 10 + 10 = 90 + 10 = 100\]
\[57 + 10 + 10 = 67 + 10 = 77\]
\[100 - 10 - 1 = 90 - 1 = 89\]