Christmas means baking, and baking means recipes. If you search for them in English, chances are you come across a delicious-sounding cake with instructions in units you aren’t accustomed to: Fahrenheit instead of Celsius or vice versa.
To convert from Fahrenheit to Celsius, you need to subtract 32, multiply the result by 5 and divide by 9. The conversion from Celsius to Fahrenheit requires the opposite: multiply the temperature by 9, divide by 5 and add 32.
The conversion formulae aren’t mental maths friendly, so you might have heard about the famous conversion trick. If you see a temperature in Fahrenheit, subtract 30 and divide the result by 2. If, for some unexplained reason, you want to change Celsius to Fahrenheit, multiply the temperature by 2 and add 30.
These shortcuts work really well when we’re talking about temperatures outside. However, the difference between 
For example, most recipes will suggest baking the cake at 180°C. Using the trick, we’d need to multiply it by 2 and add 30, which would give us a whopping 390°F instead of 356°F. You’d definitely burn your cake.
Always remember about the shortcomings of the shortcuts you use!
Certain about Uncertainty 