Cecil wouldn’t have been stumped by the question of where the Celsius and Fahrenheit temperatures coincide if he could just remember that shifting by 18 degrees Fahrenheit is the same as shifting by 10 degrees Celsius.
But for those who don’t want to have to remember the number 18, just remember any two points on the scale and you can work it out. You probably already know one:
0 Celsius = 32 Fahrenheit = freezing
Another easy to remember point is this:
10 Celsius = 50 Fahrenheit
So from that, you can easily see that an 18 degree difference on one scale is 10 degrees on the other.
Hence by a few steps of subtraction:
-10 C = 32-18 F = 14 F
-20 C = 14-18 F = -4 F
-30 C = -4 -18 F = -22 F
-40 C = -22 - 18 F = -40 F
The convenient thing about using 10 C = 50 F as your second point (besides the fact that they’re nice round numbers) is it’s not too far away from most of the temperatures you’re likely to encounter in day-to-day life. You can easily rattle off 20 C = 68 F or 30 C = 86 F
When we first went metric in the 1970s the rule of thumb was to take the Celsius temperature, double it, and add 30 to get the approximate equivalent Farenheit temperature.
