Adding and taking away within 18 · Usually met around P1
Fact families
Know one, write the family's four.
How to do it
Method
Know one, write the family's four
3 + 8 = 8 + 3 = 11, 11 − 3 = 8, 11 − 8 = 3
Another way
Check by adding
11 − 3 = 8? 8 + 3 = 11, right
In full
Three numbers can make a number family, such as 3, 8 and 11: two additions, 3 + 8 = 11 and 8 + 3 = 11, and two subtractions, 11 − 3 = 8 and 11 − 8 = 3. Swap the two numbers of an addition and the answer stays the same; a subtraction can't be swapped: 11 − 3 works, but 3 is too small to take 11 from. Know one and you can write the other three. So you can check a subtraction by adding: 11 − 3 = 8, because 8 + 3 = 11. Or look at dots: 3 and 8 together are 11, and 11 take away 3 leaves 8.
Worked examples
Have a go first, then open each one for the answer and the working.
Example 1
6 + 8 = 14, so 14 − 6 = □
Show the answer and working
Answer: 8
6, 8 and 14 are a family: 14 take away 6 leaves 8.
Example 2
2 + 8 = 8 + □
Show the answer and working
Answer: 2
Swap the two numbers of an addition and the answer stays the same: 2 + 8 = 8 + 2.
Example 3
Which of these is true?
- A. 10 − 7 = 7 − 10
- B. 3 + 7 = 3 + 10
- C. 10 − 3 = 3 − 10
- D. 3 + 7 = 7 + 3
Choose the right answer
Show the answer and working
Answer: D. 3 + 7 = 7 + 3
An addition can be swapped and keep its answer; a subtraction can't.
Practise it in the game →Adding and taking away within 18 · Free to start, unlimited questions, no download