This life-hack is significantly improved if you change it to:
C->F: multiply by 2, then add 30
F->C: subtract 30, then divide by 2.
Ill say. If I follow GPs life-hack for 30C I get 60F, which is significantly different from 86F. Some life-hack.
That's why they called it a "shitty" life hack.
30*2 + 30 = 90, which is pretty close to 86.
Going the other way
86-30 / 2 = 28 which is close to 30.
I was referring to the comment above that one.
At room temperatures, the +32 is significant, at cooking temperatures, who's oven is even accurate to 16/32 degrees? It depends on if knowing the "right" answer is more important than knowing an answer that will get you to what you want e.g., tasty muffins rather than charcoal briquettes :)
This is the correct life hack.
is approximate enough to give you an idea.
and easy enough to do mentally