Temperature Celsius vs. Fahrenheit
First things first: the fomulars
From Fahrenheit to Celsius
You have x Degrees Fahrenheit:
Celsius = (xDegreeFAHRENHEIT -32) multiplied by (5/9)
From Celsius to Fahrenheit
You have x Degrees Celsius:
Fahrenheit = (xDegreeCELSIUS divided by (5/9)) + 32
Depending on your calculation skills this task might be tricky, especially when it must be spontanuous. Assuming you don't run around with a conversion table, here comes a solution that might be not 100% accurate but fast and simple.
5/9 is exactly 0.55555555555...6, which means that we just round it to 'a half', say: 0.5
With this truncation in mind the formula gets easy to calculate and when you add in the end another 10% to your result, you get close to the exact value:
The formula simplified:
Celsius = The half of (FAHRENHEIT -32) plus 10%
An example: Let's say we have 80 degrees Fahrenheit. How many Celsius are that?
Subtract 32 from 80 (our formula says: x-32): 80-32 = 48.
Now take the half of 48 and you have Celsius: ca. 24 degrees. Just add 10% to be more accurate und you get: 26.4
using the exact formula we get: 26.6 degrees