4 ms·-30.5 degrees Celsius.by jagger27 8y ago-30.5 degrees Celsius.mark-r 8y agoAt -40 the two scales collide, you don't need to specify Celsius or Fahrenheit. Not far to go!senectus1 8y agois F logarithmic or something.?eesmith 8y agoThe conversion is F = (9/5) C + 32 . If F = C then C = (9/5) C + 32 => (1-9/5)C = 32 => C = 32/(1-9/5) = -40.