M3.2.1 For the adult
Multiplying and dividing in your head
How can I work out 25 × 16 without writing it down?
What this chapter teaches
Multiplying by 10, 100 or 1 000 moves digits left by place value, and dividing moves them right. Doubling one factor and halving the other keeps a product the same. Multiples of ten are multiplied on their non-zero digits, with the zeros put back.
Three questions to ask
There is no one right wording. Listen for the idea.
- What happens to each digit of 35 when you multiply by 10?
- Why do 25 × 16 and 50 × 8 give the same answer?
- How would you work out 60 × 70 in your head?
After the chapter, the learner can
- Multiply and divide by 10, 100 and 1 000
- Use doubling and halving to work out 25 × 16
- Multiply multiples of ten, like 30 × 40
What they copy into their notebook
To multiply by 10, 100 or 1 000, move every digit left. To divide, move them right. Double one number and halve the other. For 30 × 40, work out 3 × 4, then put back two zeros.