* bangs head on wall *
In the only interpretation of the question that makes sense, "twice as cold as 0c" is considered to be the temperature when the energy of the system falls to half its original level.
This is best described in Kelvin. It can be described in celsius or Farenheit, but that does not mean that we randomly base the calculation on "zero". Hence the bit where I said "poorly-applied physics".
If one uses any other interpretation of the question, the premise will always be flawed. Therefore whatever one does one will be "wrong", even if correctly applying the maths and/or physics.