C * 2 + 30 = ~F
(F - 32) / 1.8 = C
C * 1.8 + 32 = F
I personally find the math just as easy to do accurately. For example, 87F -32/1.8 = 55/1.8 =~30.5C. Compare that to your approximate method, which would give 28.5C, which is just wrong
(Maybe I just got really good at this when working a public facing job with a lot of American tourists - they would ask what our celsius temperatures were "in real units", so I got quite comfortable converting the air and water temps. Fahrenheit never once became intuitive to me, though.)
E.g., to convert 31℃ to ℉: 31 x 2 = 62. Subtract 6.2 = 55.8. Add 32 = 87.8℉.
If you want to round the result to the nearest integer the subtract 10% step is a convenient place: 31 x 2 = 62. Subtract 6 (rounded 6.2) = 56. Add 32 = 88℉.
When going the other way and dividing, I similarly find it mentally easier to multiply by 10/18 (rather than just divide by 1.8)
(C + 40) * 9/5 - 40 = F
Takes advantage of the fact that -40 F and -40 C are the same.
Remember it as (move origin -- convert -- move origin back).
I find it easier as a I don't have to remember precedence rules and the multiplication is obvious.
