operations · Year 4 · AC9M4A02
Turnarounds and regrouping factors
You can add or multiply two numbers in either order: 4 × 7 = 7 × 4 (a turnaround). When multiplying three numbers, you can also choose which two to multiply first. Look for a friendly pair that makes 10 or 100, such as 2 × 5 or 4 × 25. You can also split a factor to make a friendly pair: 16 × 25 = 4 × 4 × 25 = 4 × 100.
Sample questions
5 × 9 = 9 × ?
- 5 rows of 9 is the same array as 9 columns of 5: turning it round doesn't change how many dots.
- So 5 × 9 = 9 × 5. The missing number is 5.
Answer: 5
2 × 4 = 4 × ?
- 2 rows of 4 is the same array as 4 columns of 2: turning it round doesn't change how many dots.
- So 2 × 4 = 4 × 2. The missing number is 2.
Answer: 2
2 × 5 = 5 × ?
- 2 rows of 5 is the same array as 5 columns of 2: turning it round doesn't change how many dots.
- So 2 × 5 = 5 × 2. The missing number is 2.
Answer: 2