At -40 Degrees of temperature the both Celsius and Fahrenheit scales becomes same.
Here is the formulas for converting between degree Celsius and degree Fahrenheit
We know
°F = (°C * 9/5) + 32 --------------------I
°C = (°F - 32) * 5/9 --------------------II
To find the temperature when both are equal, let set ºF = ºC and solve II equations
°C = (°C * 9/5) + 32
°C - (°C * 9/5) = 32
-4/5 * °C = 32
°C = -32 * 5/4
°C = -40
