It’s a Japanese trick that uses lines to visually represent numbers
You may have heard that there’s a trick out there for visually multiplying numbers using lines. It’s a technique that they teach in Japanese schools and it works really well for simply multiplying two numbers.
Start by drawing lines to represent the digits. For example, 18 would be one line, followed by a space, followed by 8 lines. Then, take your second number and do the same thing so that the two sets of lines intersect to form a square.
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The example below multiplies 21 by 23. All you have to do is count the number of intersection points. Any intersections above each other should be added together. So for the answer, you get 4 for the first (leftmost) digit, 2 + 6 for the second digit and 3 for the third digit. So, 21 x 23 = 483.
If any of the digits add up to more than nine, simply carry the number as you would in regular multiplication. Let’s try something harder: 231 x 213
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Our first digit is 4.
Our second digit is 2+6=8, so we have 48…
Our third digit is 6+3+2=11, so we carry 1 and have 491…
Our fourth digit is 9+1=10, so we carry 1 again and get 4920…
Our last digit is 3, so we end up with 49203.
How does it work? Each section of lines represents a power of 10 (1, 10, 100, etc.). The setup is actually very similar to the one you would use in regular multiplication. You multiply each digit in the first number by every digit in the second number, which is what you are doing by counting the number of intersections. Adding the numbers above and below each other is the same as totaling the columns in regular multiplication.
Now it’s time to practice and show your friends!
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