\[\boxed{\mathbf{83.}}\]
\[280 + 1 = 281\]
\[280 + 100 = 380\]
\[630 + 1 = 631\]
\[630 + 100 = 730\]
\[420 - 1 = 419\]
\[420 - 100 = 320\]
\[570 - 1 = 569\]
\[570 - 100 = 470\]