Temperature conversions between different scales can get pretty confusing at times. If you are used to using the Celsius scale to express your temperatures, you will most probably be thrown off the scale when the units are expressed in a Kelvin/Fahrenheit scale. Addressing the question of the temperature at which the Celsius scale equals the Fahrenheit scale might sound as a very simple question, and indeed it is. However, someone somewhere is up cracking his/her brains to find the answer to this simple question.
Well, given that 1°C = 1.8°F
And that, at 0°C, the temperature of water is 32°F
Now, let the freezing point of mercury be X°C = X°F
At temperature T1; 32°F = 0°C
And at temerature T2; X°F = (X-32)/1.8
Therefore; 1.8X = X-32
Solving for X gives (negative)40
The Kelvin scale is equal to the celcius scale at -40°C = -40°F.
There you have it.