Easy Celsius to Farhrenheit conversion you can do in your head
Double the Celsius number, subtract 10%, then add 32. Result is the Fahrenheit number.
Example: 17ºC: twice 17 is 34; subtract 3.4 to get 30.6; add 32 to get 62.6: 62.6ºF, which is correct.
The reason it works is that it’s simply a restatement of the conversion formula F = 9/5 C + 32
You double the number and then subtract 10% of the doubled number, which amounts to 2x – (.1*2x).
Since .1*2 = .2, that’s the same as 2x – .2x. That’s the same as (2 – .2)x, which is 1.8x
1.8 is the decimal representation of 9/5, so you can see that the mental conversion uses the exact conversion formula. The result is not an approximation, but the actual conversion.