Remember the easy round estimates for common numbers. 30F/0C is the freezing point, room temperature is around 70F/20C, very hot weather is 100F/40C, boiling point is 210F/100C, -40 is where they both converge.

It’s actually 32/0, 68/20, 100/37, and 212/100, but these are close enough. The actual conversion is C=(F-32)/95, or F=C5/9+32