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You multiply °C by 1.8 and add 32 = °F. If you don’t have a calculator / smartphone, you double it and subtract 10%.
Ex. (°F to °C):
100°C *1.8 = 180+32 = 212°F = boiling point of water.
100*2–10% +32; 100*2=200–10% (20) =180, +32 = 212°F
°F to °C :
212 - 32 = 180. 180 / 1.8 =
