Praise then darkness, and creation unfinished

I'm shopping for a new parka, which at the moment involves looking at catalogs. The L.L. Bean catalog indicates what temperatures they think their coats are good down to (for light and moderate activity), and I was sitting looking at the catalog, doing math in my head, because the numbers are listed in Fahrenheit, and I think about Montreal weather in Centigrade. (I'll be wearing it here too, but the conditions are less extreme here.)

Since the coat I was looking at had -40 as the moderate activity rating, the easy thing to do was to work from there: how many degrees F is it from -40 to the light activity numbers, divide by 9, multiply by 5. Add that to -40. And no need to deal with 32s or slide around the Fahrenheit zero. (Yes, I have assorted machines that would do the job, as fast, but first I'd have had to grab one and open the appropriate program.) Conclusion: the coat in question will do, if I don't find another I like better.