When I tutored elementary school math, one of my favorite things to show the kids was how to multiply by 9 using their fingers. (I apologize, but this trick assumes that you have five fingers on each hand. Later you’ll see how to adapt it if you have nonstandard hands.)
Here’s how the finger rule works. Let’s say you want to multiply 4 by 9. First, put your hands in front of you with your palms away from you.
Prepare to multiply!
Count four fingers from the left, and bend that finger down. (This should be your left index finger.)
My fourth finger from the left is down. There are three fingers to its left and six to its right. Coincidence? I think not!
There are 3 fingers to the left of that finger and 6 to the right, and 4×9=36. To multiply 8 by 9, count 8 fingers from the left (this should be over to your right middle finger). There are 7 fingers to the left of it and 2 to the right, and 8×9=72.
8×9=72.
Recently, Numberphile posted a video called Pi and Four Fingers, which includes a portion of an interview with Simon Singh, author of the recently published book The Simpsons and Their Mathematical Secrets. In the video, Singh notes that even though almost all of the characters on the Simpsons have eight fingers, they use base* ten anyway. Sadly, the Simpsons can’t use the finger rule I use to multiply by nine. But if they were willing to think in base eight—as eight-fingered creatures, this would be the “natural” base for them—they could use the finger rule to multiply by seven.
Read the full post at Roots of Unity to learn how to multiply by seven on your fingers.
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