B2.1 Turn-around facts and fact families
You can add in any order: 4 + 4 is the same as 4 + 4. Adding and subtracting are partners — one undoes the other.
Example
Make a fact family from 4, 4 and 8.
- 4 + 4 = 8
- 4 + 4 = 8
- 8 − 4 = 4
- 8 − 4 = 4
Tip: To check a take-away, add back: if 8 − 4 = 4, then 4 + 4 should be 8.
B2.2 Facts to 10
Facts to 10 are the ones we want to know by heart. Practise them with fingers and with counters until they are quick.
Example
4 + 4 = ? and its partner fact.
- Count on from 4: 5, … up 4 more. 4 + 4 = 8.
- Partner take-away: 8 − 4 = 4.
B2.3 Mental math: make a ten
When adding gets near 10, make a ten first. Ten is easy to add to.
Example
Add 9 + 4 in your head.
- 9 needs 1 more to make 10.
- Take 1 from 4. That leaves 3.
- 10 + 3 = 13.
Tip: Estimate first: 9 + 4 is about 10 + 4, so the answer is close to 14.
B2.4 Adding and taking away to 50
For bigger numbers, use tens and ones. Add the tens, add the ones, then put them together.
Example
A class has 27 crayons and gets 16 more. How many now?
- Write it: 27 + 16 = ?
- Add the tens and the ones, then join them: 43.
- Check by taking away: 43 − 16 = 27. ✓
- Read the story.
- Decide: joining (add) or taking away (subtract)?
- Write the number sentence.
- Solve with tens and ones.
- Check with the partner fact.
B2.5 Equal groups and halves
Example
Put 8 crayons into 2 equal groups.
- Deal them out: one here, one there.
- Each group has 4. That is one half of 8.
- Check: 4 + 4 = 8.
Summary
- Add in any order; subtraction undoes addition.
- Know facts to 10 by heart.
- Make a ten to add in your head.
- Equal groups share fairly; half is one of two equal groups.