Split each multiple of ten into a basic fact and a power of ten: 30 × 40 is 3 × 4 = 12, and there are two zeros, so 1200. Keep any zero the basic fact makes: 50 × 60 is 5 × 6 = 30 with two more zeros, so 3000. To find a missing factor, take the same number of zeros off both numbers and divide.
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Equal groups
3 × 4 = 12
12 ÷ 3 = 4
The same picture shows multiplication and division. Each group must have the same number.
Words to know
- multiple of ten
- a number in the ten times table, such as 30 or 700
- basic fact
- a times-table fact such as 3 × 4 = 12
Worked examples
6 × 700 = ?
- 1.Basic fact: 6 × 7 = 42.
- 2.700 has two zeros, so 4200.
= 4200
50 × 60 = ?
- 1.Basic fact: 5 × 6 = 30.
- 2.Add two more zeros: 3000.
= 3000
? × 30 = 2400
- 1.Take one zero off both numbers: ? × 3 = 240.
- 2.240 ÷ 3 = 80. Check: 80 × 30 = 2400.
= 80
For parents
Method: Basic fact, then the zeros
What to say: What is the basic fact? How many zeros do the numbers have altogether?
Common mistakes:
- Multiplies the basic fact and leaves off all the zeros.
- Puts on one zero too few (often missing the zero that comes from the basic fact, as in 5 × 6 = 30).
- Puts on one zero too many.
- Finds a missing factor by subtracting instead of dividing.