This is a straight forward application of Charles' law which states that for a fixed volume of gas, the pressure is proportional to the (absolute) temperature. i.e. P/T = constant.
You have a fixed volume of gas. You know the starting conditions. You know the final pressure. It only remains to work out the temperature using P1/T1 = P2/T2
That will give you the AVERAGE temperature of the system. Half of the system is at 0 deg C (273.15 K) so you can easily work out the temperature of the other half that will give you the calculated average.
(273.15 + unknownT)/2 = Average
rearranging gives
unknownT = (2 x Average)/273.15