\[\boxed{\mathbf{23.}}\]
\[\textbf{а)}\ 27 + 1 = 28\]
\[34 + 1 = 35\]
\[51 + 1 = 52\]
\[68 + 1 = 69\]
\[\textbf{б)}\ 39 + 1 = 40\]
\[59 + 1 = 60\]
\[88 + 1 + 1 = 89 + 1 = 90\]
\[98 + 1 + 1 = 99 + 1 = 100\]