2 ms·

(F - 30) / 2 = ~C C * 2 + 30 = ~F

• SECProto 9mo ago
  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 9mo ago
    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 9mo ago
      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)
• bsder 9mo ago
  (F + 40) * 5/9 - 40 = C (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.