Idea 17of 24Builds onPartial products in a column
The algorithm in two rows
Why does the written method have only two rows?
Today you'll discover
- which parts each row joins
- why the second row ends in 0
- how both ways agree
Think first23 × 14: four lines, or two rows?
The algorithm writes 23 × 14 in two rows. The first is 23 × 4 = 92. What is the second?
Decide first, then press Next. Match each row to two of the four lines.
On the left, 23 × 14 with its four partial products 12, 80, 30 and 200. Brackets join 12 and 80 into 92, and 30 and 200 into 230. On the right, the same multiplication in two rows: 92 for 23 × 4 and 230 for 23 × 10, its last 0 ringed. Both columns add to 322. The last step shows 36 × 12.
See the whole ideathe big idea and the key word, in one place
The big idea
The algorithm's two rows are the four partial products, added in pairs.
Compare themFour parts: 12, 80, 30, 200. Two rows: 92 and 230. Same multiplication, same total, 322.
Easy to get wrongForget the zero in 230 and you get 92 + 23 = 115, far from 322.
Key word
- algorithm
- a set of steps that always works, like the two-row way to multiply
Ready to use it?
- Multiply two-digit numbers in two rows.
- Find the mistake when a zero is missing.