The formula for conversion of Fahrenheit temperatures to Centigrade is

 

F = (9 / 5)C + 32.

 

Note that 9 / 5 is equal to 1.8.

 

This can be written as

 

F = 2(1 - .1)C + 32.

 

This means: Double C, subtract 10%, and add 32. E.g., if C is 35, then double C to get 70, subtract 7, which is 10%, and get 63. Add 32, to get 95. This way one can quickly convert from C to F without using a calculator.

 

Solve for C. We get

 

(5 / 9)(F – 32) = C.

 

Using the equality axiom, a = a, we can write this as

 

C = (5 / 9)(F – 32).

 

This has C on the left, as required by mathematical convention.

 

We can proceed further, and write this as

 

C = (5 / 9)F -160 / 9.

 

This is actually more difficult to use in practice when trying to convert from F to C.