The post about the math mistake in temperature conversation reminded me of a formula that a friend told me about (thanks DSD!). She was traveling abroad, and the guide she was with said that to convert Celsius to Fahrenheit people should use the formula:
Double the Celsius, and subtract from it the amount obtained by moving the decimal place one unit to the left. Then add 32 to get the corresponding Fahrenheit.
For example, with a temperature like 50°C, you’d double 50 to get 100, then from that subtract 10.0 to get 90. Finally, you’d add 32 to 90 to get 122°F.
This is equivalent to the formula
Temp in °F = (9/5) (Temp in °C) + 32.
In particular, if C is the temperature in Celsius, the description to double and the subtract that amount with the decimal place moved describes 2C – 0.1(2C), which is 1.8C, or 9/5C.
It does seem to me to be quicker to compute 9/5C by doubling C and subtracting a tenth of the result than to multiple by 9 and divide by 5 in some order. The conversion isn’t as quick as “Double and add 30″*, perhaps, but unlike that estimation it has the advantage of being exact.
*a formula that always brings to mind the movie Strange Brew
The thermometer is by Bernard Gagnon – Own work, CC BY-SA 3.0. It has Centigrade rather than Celsius at the top, which I found interesting since I remember learning both terms in school.