How can you multiply using intersecting straight lines?
As you know, I am a bit of a Japanese abacus fanatic. This lead me to a fascinating method of multiplying using straight lines. Here’s how it works.
We’ll take an example of 123 x 332
Draw parallel lines representing the number 123
Next draw a similar set of lines in the other direction representing 332
Now, all you have to do is separate the groups of numbers as I have done below
Count the intersections in each group.
This gives you 3 9 17 13 6
Carry the 10 of the 17 left to add it to the 9 and leave 7.
Carry the 10 of the 13 to the left and add it to the 7 giving 8 and leaving 3
And finally where the 9 became a 10, carry that to the left and add it to the 3 to make 4 leaving 0
Check it with your calculator, although who need a calculator now?
There are added techniques. Notably number between 5 and 9 in the starting sum is drawn with a different colour representing the 5, just to reduce the number of lines that have to be drawn.