Monday Maths: Chisenbop

Todays fun fact is about a Korean method of finger counting called Chisenbop.  As well as having a very cool name, Chisenbop is pretty useful for doing maths when you don’t have a calculator.  Some of my Brownies have to do mental arithmetic at school.  I guess this is a pretty neat way of semi-cheating.

Counting

The basic principle of Chisenbop is how to count.  You can represent the numbers from 0 to 99 just using your ten fingers.  Here’s how:

  • The right hand is units, the left hand is tens.
  • Hold both hands out, hovering over a table, and “switch on” each finger by touching the surface of the table with it.
  • The index finger of your right hand is “one”, index and second are “two”, three fingers is “three”, four fingers is “four”.
  • The thumb is “five”; thumb and index is “six”, thumb and two fingers is “seven”, etc.
  • When you get to ten, start with the same system on the left hand – index finger is “ten”, two fingers is “twenty”, etc.

So this is forty six:

20131201-185154.jpg

This is eighty three:

20131201-185229.jpg

What’s this one?

20131201-185259.jpg

Some Neat Tricks

Now that you know how to count, there are some funky things you can do easily.

Addition & Subtraction

Addition and subtraction are mostly a case of knowing the counting system well and counting your extra numbers.  So, to add 28 and 7, I make my hands look like this:

20131201-185328.jpg

and then I count on 7 extra fingers and see where I end up.

If I want to add two “big” numbers, like 28 and 46, I can do it by adding the tens and units columns separately.  So I would start with 28, add 4 to my left hand (tens), and then add six units.

It sounds slightly convoluted and not terribly helpful, but I’m told that if you are used to it, it can be faster than finding your calculator.

Video of a cute kid: click here.

Multiply by Nine

Chisenbop is all about doing maths with your fingers.  This one doesn’t use the same counting system as I’ve been describing so far, but it still counts (see that joke I made there?) as chisenbop, because it uses your fingers.

It’s very easy to multiply by nine.  Hold out all ten of your fingers.  Say you want to do 6 x 9.  Counting from the left, fold down the 6th digit you get to – in this case your right thumb.

Now, how many digits are there before the folded one?  That’s your tens.  How many are there after?  That’s your ones.  So with my right thumb folded in, I have all five digits on my left hand, followed by four fingers on my right hand.  So my answer is 5×10 + 4 = 54.

Try a few others:

20131201-185349.jpg
3 x 9 = 2×10 + 7 = 27
20131201-185416.jpg
9 x 9 = 8×10 + 1 = 81

Multiply numbers between 5 and 9

This one’s kind of neat.

Imagine that each of your hands has fingers which represent the numbers 5 to 9.  To multiply 6 by 8, match the “six” finger on one hand to the “eight” finger on the other:

20131201-185441.jpg

Now, the fingers above the joined ones: how many are there?  There are four, so my answer is going to be 40-something.

Now take the fingers you haven’t used yet.  You should have four on one hand and two on the other.  Multiply those together (which should be easier, since they’ll be small numbers).  You get 8.

So, 6 x 8 = 48!

Simple, yet effective.

What strategies do you use for counting on your fingers?

Answers:
The picture represents the number 13.

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