Idea 6of 24Builds onSplit a fact into two you know
Nines from tens
How does 10 × 7 help you find 9 × 7?
Today you'll discover
- why 9 rows is one row short of 10
- what to take away
- how 11s work from 10s
Think first9 rows of 7 dots
10 × 7 = 70. What is 9 × 7?
Decide first, then press Next. Find the nearby ten fact, then fix it by one row.
9 rows of 7 dots. Then a dashed ghost row of 7 appears under them, crossed out, with 10 × 7 = 70 and take away a row on the right. Then 70 − 7 gives 9 × 7 = 63. The last step shows 9 rows of 8.
See the whole ideathe big idea and the key word, in one place
The big idea
Use a nearby fact you know, then add or take away the difference.
11s and 12s11s and 12s work from 10s too: 11 × 6 = 60 + 6 = 66, and 12 × 6 = 60 + 12 = 72.
Easy to get wrongTake away a whole row, not 1: 9 × 7 = 70 − 7, not 70 − 1.
Key word
- compensation
- using a nearby fact you know, then fixing the difference: 9 × 7 = 10 × 7 − 7
Ready to use it?
- Find a 9s fact from a 10s fact.
- Pick the strategy that makes a fact easiest.