Saturday, September 22, 2012

How to Multiply by Nine on Your Fingers

Back when you were six or seven, you may have learned how to do some simple addition and subtraction on your fingers. Like for 2 + 2, you might hold up two fingers on one hand, two on the other, and then add up the fingers. For 5 - 3, you could hold up five fingers, put three down, and add up the fingers left.

You may have also learned a multiplication trick on your fingers. Specifically, how to multiply by nine. Though this is very popular, I thought it was worth a cool math stuff post.

Let's solve 9 x 4. To do this, hold up your ten fingers. Then, put your left index finger (four from the left) down. On the left side, you have 3 fingers up. On the right side, you have 6 fingers up. Therefore, the answer is 36.

You can do this with other objects as well. Pretend you have nine toothpicks.


For 9 x 1, you just leave it. For 9 x 2, you bring one over to the left.

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There is one on the left and eight on the right, giving us 18. For 9 x 3, bring another over.

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You can continue this process all the way through to 9 x 10.


Now, you can actually continue to 9 x 11. Just add nine more toothpicks to the right side.

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For 9 x 12, you would have ten on the left and eight on the right, but that still is 108. This process actually works forever.

This isn't the coolest thing in the world. However, I like it because it is simple, even at a second grade level. I always say that around fifth or sixth grade, people lose interest in math. However, to lose interest, you must get an interest in the first place. Something like this can spark that interest, and some of the more complicated patterns and proofs can be taught in later years to maintain this interest.

Answers: Here are the answers to August's problem of the week. I will continue the problem of the weeks in June 2013.

x = 0%
y = 100%
g = 6
s = 5
m = 2
b = 80
t = 40
n = 120
h = 5

a = 37.5%
b = 62.5%
x = 0%
y = 20%
z = 80%
f = 1
g = 100
s = 61 cm
l = 90%
n = 95
p = 330 cm

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