Paul Tomblin wrote:
In a previous article, "kage" said:

double the C
subtract 10%
add 32

Exact


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?

The formula is:
F = C*9/5 + 32
C = (F - 32) * 5 /9;

Kage's procedure is the same:
F = (2*c) - .1(2*c) +32 (which is the same as)
F = (1.8) * c + 32 (and also)
F = 9/5 * c + 32