Friday, September 15, 2017

For Something Totally Different - Casting Out Nines

My son-in-law sent me this video.  It's always cool to learn new tricks, especially when they are 'simple.'  I've put quotes around that because this trick is simple up to a point.  The practical part.  The explanation of why it works might need a couple of reviews to catch.

Trust me here and just start watching this.  If you don't like it, just stop.  But you won't stop if you have any curiosity at all.

He does say that people used this trick back in the old days, before calculators. Well, I learned arithmetic before there were cheap, readily available handheld calculators, and I don't remember anyone ever mentioning this.

He explains why the nines don't matter by giving the example of there being seven days in the week so the sevens don't matter.  But he doesn't exactly say that we're just using nine digits (which I guess get us back to zero which also doesn't matter).  I'm assuming that's the link to seven days of the week.    Anyone know if that's right?

Also, anyone know where he comes from?  I'm assuming UK, but what part?  Listen to how he says Monday for example.  That must narrow it down for people who know British dialects.


  1. It's a cute trick, but not really very reliable. You can be off by a factor of 9 (or 90 or 900) and the digital counter would still appear to be "correct".
    I was in engineering school at about the time when electronic calculators came on the market. There are a lot of ways we would check our calculations, but I never knew anyone who would rely on this method.
    However, if you want to know some interesting tricks to use to perform or check calculations, try the methods developed by Jakow Trachtenberg. You can find some of his methods on Wikipedia:

  2. Hey Steve, good to hear from you. Yes, he does mention the fact that you could be off by nine, but I thought the implication was that that would be significant enough one would notice the error.
    But the Trachtenberg stuff is fascinating too. I looked him up and he came up with these techniques for doing these calculations while he was imprisoned in German concentration camps. I've always hoped that were I to be in a prison like situation, I'd be able to occupy myself totally in my head like that, to use my imagination to keep being active and utilize the time well. Though I realize those conditions come with other physical problems like hunger, diseases, pain that makes such mental escape even harder. He would have been in his 50s during WW II.
    And I continue to want to challenge my commenters with links to follow the click here link in my notice above the comment box to learn how to actually link the url's the refer others to. For instance, here's the link you offered:


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