For Fahrenheit to Celsius conversion, you only need to calculate with twos:
Divide by two
Add two
Subtract two from the tens (i.e., subtract 20).
For extra accuracy, after adding two (in step two 😉), slide that number over to the right (i.e., divide by 10) and add that on. Do this as many times as you are comfortable with (which can be none for rough calculations).
For example:
90ºF divided by two is 45
45 plus two is 47
Subtracting two from the 4 in 47 leaves 27
This is quite rough so round it off: 30ºC
More accurately:
90ºF divided by 2 is 45
45 plus two is 47
Sliding 47 over to the right makes 4.7
47 plus 4.7 is 51.7
Subtracting two from the 5 in 51.7 leaves 31.7
This is a little rough so round it off: 32ºC.
Probably unnecessarily accurate:
90ºF divided by 2 is 45
45 plus 2 is 47
Sliding 47 over gives 4.7, and again is 0.47
47 + 4.7 + 0.47 = 52.17
Subtracting twenty makes 32.17ºC
There’s still an infinity of sliding that could go on so, once again, round off: 32.2ºC.
The usual (F - 32) * 5 ÷ 9 formula gives 32.222…ºC.
Yeah, that’s why I can do it, because it’s simple as all hell haha. I really am terrible at math, you have no idea. Absolute doofus level.
For Fahrenheit to Celsius conversion, you only need to calculate with twos:
For extra accuracy, after adding two (in step two 😉), slide that number over to the right (i.e., divide by 10) and add that on. Do this as many times as you are comfortable with (which can be none for rough calculations).
For example:
This is quite rough so round it off: 30ºC
More accurately:
This is a little rough so round it off: 32ºC.
Probably unnecessarily accurate:
There’s still an infinity of sliding that could go on so, once again, round off: 32.2ºC.
The usual (F - 32) * 5 ÷ 9 formula gives 32.222…ºC.