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
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
For C to F, double it, subtract a 10th of the result, add 32.
E.g. 20 double is 40, minus a 10th is 40-4, so 36, add 32 giving 68.
If you ignore fractions when you take the tenth, you can do it in your head to be pretty close.
17, x2 is 34 a 10th is about 3, so 34-3 is 31, +32 is 63 (actually 62.6).