Celcius to Farenheit: multiply by 9/5, add 32 (100°C x 9 = 900. 900 / 5 = 180. 180 + 32 = 212°F)
Farenheight to Celcius: subtract 32, multiply by 5/9 (212°F – 32 = 180. 180 / 9 = 20. 20×5 = 100/°F).

Plug in your numbers
-40°F – 32 = -72. -72 / 9 = -8. -8×5 = -40°C
-40°C * 9 = -360. -360 / 5 = -72. -72 + 32 = -40°F

(They don’t meet at 40° above zero. 40°F is 4 4/9°C. 40°C is 104°F)

Oh my mistake about above. I guess this question is useless now. I’ll let it stand as a testiment to my stupidity, Unless I can delete it.

Cue the mods :) thx in advance

No stupidity. Just misinformed. Very different.

LOL Yeah I was just kidding :)

They meet at minus 40 only. As linear scales they can only meet at one point.

Math

Oh, and only at -40°F and -40°C

If you had a centimeter ruler and an inch ruler, you would normally line them up matching the zeros.

The Celsius and Fahrenheit scales are like that, but lined up at -40 instead of 0.

In these parts, -40, on any scale, is damned cold!