Number families
You learn to make four facts from three numbers that belong together in a fact family.
How to work it out
- With three numbers you make four sums.
- Turn the numbers around for plus.
- Take away from the total for minus.
A worked example
7, 3 and 10
- Make the first addition sum: 7 + 3 = 10.
- Flip the numbers for the second addition: 3 + 7 = 10.
- Take away from the total for the subtractions: 10 − 3 = 7 and 10 − 7 = 3.
More examples
8, 5 and 13
- Add: 8 + 5 = 13 and 5 + 8 = 13.
- Take away from 13: 13 − 5 = 8 and 13 − 8 = 5.
25, 15 and 40
- Add: 25 + 15 = 40.
- Take away from the total: 40 − 15 = 25.
Try it yourself
Finish the sum using the number family of 8, 5 and 13
5 + 8 = ?
Finish the sum using the number family of 9, 8 and 17
17 − 9 = ?
Watch out
In a subtraction, you take the two small numbers away from each other instead of from the total.
Always start the subtractions with the largest number: the total.
Tips
- The family always has two addition sums and two subtraction sums with the same three numbers.
Why it works
Addition and subtraction go together. When you know that two numbers make a whole, you know four facts straight away. That is how you turn addition facts into subtraction facts.