Double the number, then reduce by one-tenth the doubled number, then add 32
Example: To convert 25 C double it to 50, reduce by 5 to get 45, then add 32 to get 77 F.
The reason this works is because 9/5 = 10/5 - 1/5. And the nice thing about this is that it is an exact conversion not an estimation.
Works just the same:
-10 C doubles to -20, reduce by -2 to -18, and add 32 to get 14 F.
I think I was getting confused with the “reducing” a negative number. But yeah reducing a negative negatively moves it positively