Today I learned that my co-worker figures a tip by multiplying by .2 on her cell phone (calculator function).
She was a bit dumbfounded when I pointed out that it was easier for me to multiply by 2 and then divide by 10, or vice versa, than to take my phone out of my pocket. She IS of an age where she can multiply by two in her head with no trouble, it just never dawned on her that this was usable to figure 20%. I then showed her how she could use division by two and an addition to figure a 15% tip for just OK service.
This got into a bit of discussion of similar shortcuts. One of my favorites is multiplying by 1.8 (for C-F temperature conversion) by first doubling, then subtracting 10% of that product…this I can do in my head, but not “real” multiplication by 18.
I frequently need to convert between inches and millimeters. Knowing that 1mm is very close to .040 inches is very handy, because multiplication and division by 4 is pretty easy, sometimes I do resort to two divisions/multiplications by 2 though.
Basically, 4 is a much handier number for top-of-my head conversion than 25.4 While the later is exact, the former gets me close enough that I know I need a 10mm wrench when the 3/8" won’t quite fit over the bolt head.
Recently, on these boards, a metrication proponent cited the convenience and logic of a system where 1 liter of water has a mass of 1Kg…just another of the many people that don’t seem to realize a pint of water weighs exactly a pound* I don’t need to use this much, but it does make it trivial to remember that water density is 8 lb/gal…which I do use a lot.
Finally, it seems that most folks (the ones who can do it at all) figure “price with tax” by calculating price * tax_rate + price. Which is how I’d still do it in my head, or with pencil and paper. Using a calculator, though, this requires punching in the price twice. I find it a little easier to figure as: price * (1+tax_rate), as the addition of 1 can be done in my head. When I was working retail in college, this one completely mistified one of my managers “Kevbo does it wrong, but still gets the right answer!”
So what little tricks or approximations do other dopers use to make simple calculations even simpler?
** yet will still refer to 16 oz. beer bottles as “pounders”, presuming this refers to “pounding them down”
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