Stage 6 · Big numbers & speed
×9 and ×99
×99 is ×100 take away one lot.
How to do it
Method
×99 is ×100 take away one lot
47 × 99 = 4700 − 47 = 4653
Another way
Column multiplication
47 × 9 = 423, 47 × 90 = 4230, 423 + 4230 = 4653
In full
×9 is ×10 take away one lot: 47 × 9 → 470 − 47 = 423. ×99 is ×100 take away one lot: 47 × 99 → 4700 − 47 = 4653. That's the distributive property: 47 × 99 = 47 × 100 − 47 × 1. You can also multiply in columns; the answer is the same.
Worked examples
Have a go first, then open each one for the answer and the working.
Example 1
31 × 9
Show the answer and working
Answer: 31 × 9 = 279
31 × 9: ×9 is ×10 take away one lot, 31 × 10 = 310, 310 − 31 = 279
Example 2
15 × 99
Show the answer and working
Answer: 15 × 99 = 1485
15 × 99: ×99 is ×100 take away one lot, 15 × 100 = 1500, 1500 − 15 = 1485
Example 3
50 × 99
Show the answer and working
Answer: 50 × 99 = 4950
50 × 99: ×99 is ×100 take away one lot, 50 × 100 = 5000, 5000 − 50 = 4950
Example 4
34 × 99
Show the answer and working
Answer: 34 × 99 = 3366
34 × 99: ×99 is ×100 take away one lot, 34 × 100 = 3400, 3400 − 34 = 3366
Practise ×9 and ×99 in the game →Stage 6 · Big numbers & speed · Free to start, unlimited questions, no download