Mental Calculation for Temperature Scales

Today, I heard the first cry of cicadas. It's summer! And It is very hot. The maximum temperature is 30 deg. Celsius. But how do you say it in Fahrenheit?
I am sometimes confused with the two scales of temperature: Fahrenheit and Celsius.


You know Fahrenheit is used in a few countries including the U.S. I don't know why Fahrenheit is still commonly used in the U.S. even though many other countries use Celsius. Every time I talk about weather with them, I have to "translate" Celsius to Fahrenheit, and vice versa.

But it is ridiculous to always hold a pen and notes (or a calculator) to use this formula: (F=C*9/5+32). So I have several steps to easily calculate scales in my head.

Celsius to Fahrenheit:
You have a value C in Celsius.

1. Double C
       C * 2
2. Subtract 10% of it
       2C - 0.1 * 2C
3. Add 32
       2C - 0.1 * 2C + 32

Then you get an exact value in Fahrenheit.

It is easier than expected.
For example, let's say the temperature is 30 deg. C.
So you can calculate as the below:
   30*2=60
   60-6=54
   54+32=86 The temperature is exactly 86 deg. F.

In case of 20 deg. C.:
   20*2=40
   40-4=36
   36+32=68 The temperature is 68 deg. F.

The reason can be derived from the formula.
F=C*9/5+32
=C*2/2*9/5+32
=C*2*(9/10)+32
=C*2*0.9+32
=C*2-C*2*0.1+32


Fahrenheit to Celsius:
You have a value F in Fahrenheit.

1. Divide F by 2
      F/2
2. Subtract 16 from it
      F/2 - 16
3. Add 10% of it
      F/2-16 + 0.1 * (F/2-16)

In this case, you get an approximately value in Celsius.

For example, let's say the temperature is 70 deg. F.
So you can calculate as the below:
   70/2=35
   35-16=19
   19+1.9=20.9  The temperature is about 21 deg. C.

The reason can be derived from the formula.
C=(F-32)*5/9
=(F-32)/2*2*5/9
=(F/2-16)*10/9
=(F/2-16)*1.11111...
≒(F/2-16)*1.1