At what temperature are Celsius and Fahrenheit equal?

A. -40 degree
B. 40 degree
C. -10 degree
D. 10 degree

The formulas for converting between degree Celsius and degree Fahrenheit are:
°F = (°C * 9/5) + 32
°C = (°F – 32) * 5/9
To find the temperature when both are equal, we use an old algebra trick and just set ºF = ºC and solve one of the equations.
°C = (°C * 9/5) + 32
°C – (°C * 9/5) = 32
-4/5 * °C = 32
°C = -32 * 5/4
°C = -40
°F = (°F * 9/5) + 32
°F – (°F * 9/5) = 32
-4/5 * °F = 32
°F = -32 * 5/4
°F = -40
So the temperature when both the Celsius and Fahrenheit scales are the same is -40 degrees.