For ideal gas, at constant volume, pressure is proportional to temperature.
Let pressure varies as, P = a × T + b , where a and b are constants
at t = 0oC , P = 60 cm of mercury column. hence we have, 60 = a×0 + b , i.e., b = 60
at t = 100°C , P = 100 cm of mercury column. hence we have, 100 = a×100 +60 i.e., a = 0.4
Hence pressure at t = 80°C , P = 0.4 × 80 + 60 = 92 cm of mercury