Adding and Taking Away: Facts to 20 and Tens

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What this lesson covers

  • Facts to 20 can be learned by heart, like 17 + 3 = 20.
  • Adding and taking away undo each other.
  • A small fact helps with tens: 7 + 2 = 9, so 70 + 20 = 90.
  • With three numbers, look for a ten first.
Key idea

Facts to 20

  • Some facts are worth knowing by heart, so you do not have to count.
  • Pairs that make 10 help a lot: 6 + 4, 7 + 3, 8 + 2.
  • Pairs that make 20 work the same way: 17 + 3, 16 + 4.
FactAlso true
6 + 4 = 104 + 6 = 10
17 + 3 = 203 + 17 = 20
8 + 7 = 157 + 8 = 15
9 + 9 = 18double 9 is 18
Key idea

Maths blocks

Key idea

Adding and taking away undo each other

  • 8 + 5 = 13. Take the 5 away again and you are back at 8.
  • So one adding fact gives you two taking-away facts.
  • Use this to check: if 13 − 5 = 8, then 8 + 5 should be 13.
Adding factTaking-away facts
8 + 5 = 1313 − 5 = 8 and 13 − 8 = 5
12 + 6 = 1818 − 6 = 12 and 18 − 12 = 6
Worked example

Doubles and halves

  • A double is a number added to itself: double 6 is 6 + 6 = 12.
  • A half undoes a double: half of 12 is 6.
DoubleHalf
double 5 = 10half of 10 = 5
double 6 = 12half of 12 = 6
double 8 = 16half of 16 = 8
double 9 = 18half of 18 = 9
double 10 = 20half of 20 = 10
Worked example

A small fact for a big sum

  • 7 + 2 = 9, so 7 tens and 2 tens make 9 tens: 70 + 20 = 90.
  • 9 − 4 = 5, so 90 − 40 = 50.
Small factTens fact
7 + 2 = 970 + 20 = 90
9 − 4 = 590 − 40 = 50
3 + 5 = 830 + 50 = 80
Worked example

Adding tens or adding ones

  • 32 + 20: add 2 tens. The tens digit changes: 52.
  • 32 + 2: add 2 ones. The ones digit changes: 34.
  • Taking away works the same way.
SumWhat changesAnswer
32 + 20tens52
32 + 2ones34
58 − 30tens28
58 − 3ones55
Key idea

Try it: hop in tens or ones

Worked example

Three numbers at once

  • 7 + 3 + 6: find a ten first. 7 + 3 = 10, then 10 + 6 = 16.
  • 4 + 5 + 6: add 4 and 6 first to make 10, then add 5. That is 15.
  • 9 + 5 − 4: 9 + 5 = 14, then 14 − 4 = 10.
SumFirstThen
7 + 3 + 67 + 3 = 1010 + 6 = 16
4 + 5 + 64 + 6 = 1010 + 5 = 15
9 + 5 − 49 + 5 = 1414 − 4 = 10

Common mistakes

  • 32 + 20 = 34
    20 is 2 tens, so the tens digit goes up: 32 + 20 = 52.
  • Half of 18 is 8.
    Half of 18 is 9, because 9 + 9 = 18.
  • 13 − 5 = 7
    Check by adding: 8 + 5 = 13, so 13 − 5 = 8.

Your turn

1

14 + 6 = ?

2

What is half of 16?

3

45 + 30 = ?

4

67 − 4 = ?

5

8 + 5 + 2 = ?

Show answer

1. 20 2. 8 3. 75 4. 63 5. 15

Summary

What to take away

  • Learn facts to 20 by heart.
  • Adding and taking away undo each other.
  • Use a small fact to add tens.
  • With three numbers, make a ten first.
Report a mistake

More in Number

Numbers to 120: Read, Write and CompareNotesGroups of Ten and Where Numbers Sit on a LineNotesRounding, Skip Counting, Odd and EvenNotesAdding and Taking Away: Facts to 20 and TensNotesStory Problems: Adding and Taking Away to 100NotesTimes and Share: 2s, 5s and 10sNotesHalves, Thirds and QuartersNotesComparing Fractions and Finding the WholeNotesNZ Coins and Notes: Count and Make MoneyNotes

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