In a previous article, Ron Natalie said:
Paul Tomblin wrote:
Add 40. Multiply by 9/5 or 5/9 depending on whether you're going F-C or
C-F. Subtract 40. Works both ways, exactly.



40? Where does 40 come from?

Try it, it works. -40 is where the two values are equal, so essentially
you're moving the graph "F versus C" so that it goes through the origin,
applying the slope of the line, and then shifting it down again.

-40 + 40 = 0
0 * 5/9 = 0
0 - 40 = -40 - -40 F = -40 C

0 + 40 = 40
40 * 5/9 = 22.22
22.222 - 40 = -17.7777 - 0 F = -17.8 C

212 + 40 = 252
252 * 5/9 = 140
140 - 40 = 100 - 212 F = 100 C

And so on.

--
Paul Tomblin http://xcski.com/blogs/pt/
"Unfortunately, you go to war with the Secretary of Defense you have, not
the Secretary of Defense you wish you had."