\[\boxed{\mathbf{243.}}\]
\[1)\ 255 + 23 = 278\]
\[255 - 23 = 232\]
\[2)\ 145 + 13 = 158\]
\[231 + 25 = 256\]
\[317 + 51 = 368\]
\[423 + 34 = 457\]
\[614 + 45 = 659\]
\[742 + 52 = 794\]
\[156 - 12 = 148\]
\[254 - 23 = 231\]
\[366 - 25 = 341\]
\[575 - 34 = 541\]
\[658 - 46 = 612\]
\[797 - 55 = 742\]