[SECProto]

No need to simplify this.

(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.)

[tzs]

For C to F you can often simplify the mental math by doing the multiply by doubling then taking off 10%.

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℉.

[SECProto]

Yes, doubling then subtracting 10% of the resultant works because that is the same as multiplying by 1.8 :)

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)