The other day I was talking to some friends about how I tend to do math in my head – oftentimes I round to get the easy answer and then remove the rounding, if you follow. So if I need to know 15%, I chop off the last digit (10%) and add half of that. They looked at me like I grew another head.

I ran into a wiki link today on a guy who does fast math in clubs, like some sorta comedian. First math trick in the comments is about squaring, and involves rounding.

What’s your favorite “math trick” that you use?

PS I hate blogs that obscure the link they’re referencing – do you click the image? the title? somewhere else? Make it obvious, eh?

~ by Skennedy on November 17, 2009.

Posted in Uncategorized

10 Responses to “”

  1. I’ve always figured out 15% like that, and I am amazed that more people don’t do it. I’m usually pretty bad at “math tricks” like that, but that’s the one I’ve found helpful on a near constant basis. Or for 20%, move the decimal point and double.

    aiela said this on November 17, 2009 at 3:32 pm

  2. Im dyslexic so my main math trick is making sure I have all the numbers in the right order before I start… :-p

    krisann said this on November 17, 2009 at 4:17 pm

  3. I do the 10%/15% thing too, and I’m also always surprised that more people don’t. But that’s the only one I can think of.

    thunderclap8 said this on November 17, 2009 at 5:41 pm

  4. How the hell else would you do it? Although lately I tend to tip 20% which is even easier.

    wendigovegan said this on November 17, 2009 at 9:16 pm

  5. I do a lot of rounding to the tens and then doing the math – meaning 49 x 3 is easily 40 x 3 plus 9 x 3. Or 49 x 6 is 50 x 6 minus 1 x 6.

    I used to look at the tax on the food bill (used to be 5%) and triple it. Or, 10% plus 5%. Seeing folks pulling out calculators always amazed me.

    meepkitty said this on November 17, 2009 at 11:00 pm

  6. I get paid to do math tricks. *laugh* Works for me.

    blazepoet said this on November 17, 2009 at 11:55 pm

  7. I do 15% the same way you do. Is that odd? I don’t think so. What probably is odd is the number of situations to which I can (entirely legitimately) apply proportions.

    ravenofdreams said this on November 18, 2009 at 2:41 am

  8. I do this too, (unless I have a particularly virulent case of the stupids, obviously,) and I have found that many people are Easily Impressed by math tricks. There’s the fast-and-dirty C to F conversion (double it and add 30,) dividing by tens to figure out sale prices (this, for some reason, *amazed* customers,) and being able to figure out tax even after it went up to 6% (divide by four, add half again, etc.)

    Then again, I also do algebra and long division in my head to pass time when I am bored. So I’m used to the strange looks by now.

    talyx said this on November 18, 2009 at 1:53 pm

  9. i do tipping the same way you do it :D

    velvettnrbt said this on November 18, 2009 at 2:16 pm

  10. I do that. I probably do more that I’m not very conscious of, too.

    rbradakis said this on November 19, 2009 at 12:16 am

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