Stage 1 · The World of Numbers / #5
What Long Multiplication Is Really Doing
Coming soon
The video is in production. Follow along on GitHub to hear when it is out.
The idea
Cut the twenty-three by fourteen rectangle into four blocks: two hundred, thirty, eighty and twelve; the second row of the working is the top two blocks, ten twenty-threes, which is why it shifts over.
A common mix-up
Twenty-three times fourteen is twenty times ten plus three times four, two hundred and twelve (the two crossing blocks get left out).
Where it stops working
Without the shift, it becomes ninety-two plus twenty-three, only one hundred and fifteen, less than ten twenty-threes.
School year
China 3–4 US 4 UK 5 Singapore 4