1.2.1

Mental Addition & Subtraction

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Adding Numbers Mentally

We can add numbers from three-digit numbers mentally. This means we work it out in our head without writing anything down.

Adding ones

Adding ones

  • To work out 463 + 5, look at the ones column:
    • 5 + 3 = 8
    • 463 + 5 = 468
Adding ones

Adding ones

  • Sometimes, it will affect two columns:
  • To work out 463 + 8, look at the ones column:
    • 3 + 8 = 11
  • It is bigger than 10, so both the ones and the tens column will change:
    • 463 + 8 = 471
Adding tens

Adding tens

  • The same rules apply for 10s, but this time we look at the tens column.
  • For example, 485 + 10 = 495.
    • Only the tens column has changed.
  • But 485 + 30 = 515.
    • Both the tens and hundred columns have changed.
Adding hundreds

Adding hundreds

  • To mentally add 100s, we look at the hundreds column.
  • 284 + 400 = 684
    • Only the hundreds column has changed.
  • But 284 + 800 = 1084
    • Both the hundred and thousands columns have changed.

Subtracting Numbers Mentally

We can also subtract numbers from three-digit numbers mentally.

Subtracting ones

Subtracting ones

  • Mentally subtracting is the same strategy but the inverse (opposite).
  • For example, 567 - 6 = 561
    • Only the ones column changes.
  • But 432 - 6 = 426
    • The tens and ones column has changed.
Subtracting tens

Subtracting tens

  • For example, 485 - 70 = 415
    • Only the tens column changes.
  • But 954 - 60 = 894
    • The hundreds and tens column has changed.
Subtracting hundreds

Subtracting hundreds

  • For example, 631 - 200 = 431
    • Only the hundreds column changes.
Jump to other topics
1

Year 3 - Number

1.1

Number & Place Value

1.1.1Multiples1.1.2Counting in Multiples1.1.3Recognising Three-Digit Numbers1.1.4Finding 10 More or Less than a Number1.1.5Finding 100 More or Less than a Number1.1.6Ordering & Comparing Numbers up to 1,0001.1.7Writing & Reading Numbers1.1.8Representing Numbers
1.2

Addition & Subtraction

1.2.1Mental Addition & Subtraction1.2.2Column Method1.2.3Estimating Calculations1.2.4Inverse Operations1.2.5Missing Number Problems1.2.6Real-Life Problems
1.3

Multiplication & Division

1.3.1Times Tables1.3.2Using Times Tables1.3.3Multiplying Numbers1.3.4Missing Number Problems1.3.5Real-Life Problems
1.4

Fractions

1.4.1Tenths1.4.2Fractions1.4.3Fractions Equal to a Whole1.4.4Equivalent Fractions1.4.5Fractions of Shapes1.4.6Fractions of Amounts1.4.7Add & Subtract Fractions1.4.8Compare & Order Fractions
2

Year 3 - Measurement

2.1

Length

2.1.1Length2.1.2Comparing Lengths2.1.3Add & Subtract Lengths2.1.4Perimeter
2.2

Mass

2.2.1Mass2.2.2Comparing Masses2.2.3Add & Subtract Masses
2.3

Volume & Capacity

2.3.1Volume & Capacity2.3.2Comparing Volumes2.3.3Add & Subtract Volumes
2.4

Money

2.4.1Money2.4.2Counting Money2.4.3Adding & Subtracting Money
2.5

Time

2.5.1Units of Time2.5.2Clocks2.5.3Telling the Time2.5.4Time of the Day2.5.512-Hour & 24-Hour Clocks2.5.6Measuring Time
3

Year 3 - Geometry

3.1

Angles

3.1.1Angles in Degrees3.1.2Types of Angle
3.2

Properties of Shapes

3.2.12D Shapes3.2.23D Shapes
3.3

Straight Lines

3.3.1Straight Lines3.3.2Parallel & Perpendicular Lines
4

Year 3 - Statistics

4.1

Statistics

4.1.1Statistics4.1.2Pictograms & Bar Charts
5

Year 4 - Number

5.1

Number & Place Value

5.1.1Counting in Multiples5.1.2Recognising Four Digit Numbers5.1.3Find 1,000 More or Less than a Number5.1.4Negative Numbers5.1.5Ordering & Comparing Numbers Over 1,0005.1.6Representing Numbers5.1.7Rounding Numbers5.1.8Roman Numerals
5.2

Addition & Subtraction

5.2.1Written Methods for Adding Large Numbers5.2.2Written Methods for Subtracting Large Numbers5.2.3Estimation & Inverse Operations5.2.4Solving Two-Step Problems
5.3

Multiplication & Division

5.3.1Times Tables5.3.2Using Times Tables5.3.3Multiplication Rules5.3.4Column Method5.3.5Factors
5.4

Fractions & Decimals

5.4.1Identifying Equivalent Fractions5.4.2Adding & Subtracting Fractions5.4.3Hundredths5.4.4Decimals5.4.5Converting Between Decimals & Fractions5.4.6Rounding Decimals5.4.7Ordering Decimals
6

Year 4 - Measurement

6.1

Units

6.1.1Metric Measurements6.1.2Adding & Subtracting Money
6.2

Time

6.2.1Converting Units of Time6.2.2Changing the 12-Hour Clock to the 24-Hour Clock6.2.3Changing the 24-hour Clock to the 12-Hour Clock
7

Year 4 - Geometry

7.1

Properties of Shapes

7.1.1Symmetry7.1.2Types of Triangle7.1.3Quadrilaterals
7.2

Perimeter & Area

7.2.1Perimeter7.2.2Area
7.3

Position & Direction

7.3.1Coordinates7.3.2Translation
8

Year 4 - Statistics

8.1

Statistics

8.1.1Types of Data8.1.2Pictograms & Bar Charts8.1.3Line Graphs

Practice questions on Mental Addition & Subtraction

Can you answer these? Test yourself with free interactive practice on Seneca — used by over 10 million students.

  1. 1
    What is 235 + 4?Multiple choice
  2. 2
    What is 560 + 50?Multiple choice
  3. 3
    What is 792 + 300?Multiple choice
  4. 4
    What is 743 - 70?Multiple choice
  5. 5
    What is 135 - 8?Multiple choice
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Representing Numbers

Column Method