Because 12C ~ 21F there are some convenient temperatures to interpolate between.

of course 0C = 32F and -40C = -40F but also (within a degree F):

  04C =  40F
  16C =  61F
  28C =  82F
  40C = 104F

note that the last two digits are always reversed.

below 0C they get less useful

  -08C =  18F
  -19C =-1.9F
  -31C = -23F

Interesting strategy. I long ago memorized 20C = 68F and then adjust in increments of 5/9, which works well enough for the range of temperatures I consider habitable. I've gotten so good at this I once translated on the fly between some fellow American and European runners during a marathon. :-)

"When it's springtime in Alaska, it's forty below". That's all I have managed to memorize about temperature conversion so far in my life.

56.7F = 13.7C