The Celsius system is almost always used by most people in France for ordinary (non-scientific) purposes.
The freezing point of water in Fahrenheit is 32 degrees, in Celsius it is 0 degrees.
The boiling point of water in Fahrenheit is 212 degrees, in Celsius it is 100 degrees.
Consequently the difference between freezing and boiling is 180 degrees Fahrenheit (212-32) or 100 degrees Celsius (100-0). This means that 180 degrees change in Fahrenheit is equal to 100 degree change in Celsius, or more simply 1.8 degrees Fahrenheit equals 1.0 degrees Celsius.
This gives rise to the following equations to convert between Celsius and Fahrenheit:
°C = (°F - 32) ÷ 1.8 For example: (68°F-32) ÷ 1.8 = (36) ÷ 1.8 = 20°C
°F = (°C x 1.8) + 32 For example: (20°C x 1.8) + 32 = (36) + 32 = 68°F