Number Facts to 10, Doubles and Adding to 20

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What this lesson covers

  • Adding 0 or taking away 0 does not change a number.
  • You can add numbers in any order and get the same answer.
  • Pairs like 7 and 3 make 10.
  • Doubles and halves go together: double 4 is 8, half of 8 is 4.
Key idea

Pairs that make 10

  • Hold up 10 fingers. Fold some down.
  • The fingers up and the fingers down always make 10.
  • Take one part away and the other part is left: 10 − 3 = 7.
PartPartTotal
01010
1910
2810
3710
4610
5510
Key idea

Try it: make 10 on a ten frame

Key idea

Adding or taking 0

  • 0 means nothing. Add nothing and you still have the same.
  • 6 + 0 = 6. 10 + 0 = 10.
  • Take nothing away and nothing changes: 9 − 0 = 9.
Key idea

Add in any order

  • 3 + 5 = 8 and 5 + 3 = 8.
  • Swap the numbers and the total stays the same.
  • So start with the bigger number: for 2 + 9, think 9, then 10, 11.
SumSwappedTotal
3 + 55 + 38
2 + 99 + 211
4 + 66 + 410
Worked example

Doubles and halves

  • A double is the same number added to itself.
  • Halving goes back the other way.
  • Double 4 is 8, so half of 8 is 4.
DoubleIsSo half of
1 + 122 is 1
2 + 244 is 2
3 + 366 is 3
4 + 488 is 4
5 + 51010 is 5
Worked example

The number before and after

  • The number after is one more.
  • The number before is one less.
  • This is adding 1 or taking away 1.
BeforeNumberAfter
678
121314
151617
181920
Worked example

Adding and taking away to 20

  • Start at the bigger number and count on.
  • To take away, count back.
  • Use facts you know: 2 + 5 = 7, so 12 + 5 = 17.
SumThinkAnswer
12 + 512, then 13, 14, 15, 16, 1717
18 − 68 − 6 = 2, so 18 − 6 = 1212
15 + 0adding 0 changes nothing15
3 + 14start at 14: 15, 16, 1717
Key idea

Try it: count on and back to 20

Common mistakes

  • Thinking 5 + 0 = 0.
    5 + 0 = 5. Adding nothing leaves 5.
  • For 3 + 8, start at 3 and count on 8.
    Start at 8 and count on 3: 9, 10, 11. It is quicker.
  • Half of 8 is 2.
    Half of 8 is 4, because 4 + 4 = 8.
  • The number before 20 is 21.
    The number before 20 is 19. 21 comes after.

Your turn

1

7 + 3 = __

2

9 + 0 = __

3

What is double 4?

4

What is half of 10?

5

16 − 5 = __

Show answer

1. 10 2. 9 3. 8 4. 5 5. 11

Summary

What to take away

  • Know the pairs that make 10.
  • Adding or taking 0 changes nothing.
  • Swap the order when adding; start with the bigger number.
  • Doubles help you halve.
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More in Number

Counting to 100 and Place ValueNotes1st, 2nd, 3rd, Te Reo Numbers and Number LinesNotesGroups, Pairs and Counting in 2s and 10sNotesNumber Facts to 10, Doubles and Adding to 20NotesJoining, Taking Away and Equal GroupsNotesHalves and QuartersNotesSharing Fairly: From Halves to QuartersNotesNew Zealand Coins and NotesNotes

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