Video: Cool Multiplication Math Trick

multiplication_with_linesThe following video will do nothing to improve your understanding of finance (my primary Bankers Anonymous goal) but if you, like me, occasionally have the opportunity to teach math to 8 years olds, you might find this as cool as I do.

I had never seen the ‘visual way’ to teach and/or do multiplication until I saw this on my Facebook feed. Also, I have no particular idea why it works, but would be happy to learn, in case any readers can shed some light.

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2 Replies to “Video: Cool Multiplication Math Trick”

  1. If you remember back to when you multiplied the old school way:
    x 31

    First, you multiplied 1 by 22 (which you could think of as 2 and then 2 in the 10s place), and wrote that down, then you shifted over a decimal place (factor of 10 – which you could fill in with a zero) and multiplied 22 by 3. Then you added the sums (each col, carrying over as needed).

    You’re basically doing the same thing here, but graphically splitting out the various places (1’s digit is group furthest to right, then 10’s, 100’s, etc moving to left) instead of lining them up as you record your multiplication by hand. There’s really no fancy explanation to it.

    Alternatively, you could think of it as computing the products of (2*10 + 2) * (3*10 + 1) = (2*3)*10^2 + (1*2+2*3)*10 + 2*1. Same thing, but that’s where the graphical representation comes from.

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I founded Bankers Anonymous because, as a recovering banker, I believe that the gap between the financial world as I know it and the public discourse about finance is more than just a problem for a family trying to balance their checkbook, or politicians trying to score points over next year’s budget – it is a weakness of our civil society. For reals. It’s also really fun for me.

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