Mental Addition & Subtraction

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

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

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

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

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

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

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

Subtracting hundreds

For example, 631 - 200 = 431
- Only the hundreds column changes.

1Year 3 - Number

2Year 3 - Measurement

2.1Length

2.1.1Length

2.1.2Comparing Lengths

2.1.3Add & Subtract Lengths

2.1.4Perimeter

2.2Mass

2.2.1Mass

2.2.2Comparing Masses

2.2.3Add & Subtract Masses

2.3Volume & Capacity

2.3.1Volume & Capacity

2.3.2Comparing Volumes

2.3.3Add & Subtract Volumes

2.4Money

2.4.1Money

2.4.2Counting Money

2.4.3Adding & Subtracting Money

2.5Time

2.5.1Units of Time

2.5.2Clocks

2.5.3Telling the Time

2.5.4Time of the Day

2.5.512-Hour & 24-Hour Clocks

2.5.6Measuring Time

3Year 3 - Geometry

3.1Angles

3.1.1Angles in Degrees

3.1.2Types of Angle

3.2Properties of Shapes

3.2.12D Shapes

3.2.23D Shapes

3.3Straight Lines

3.3.1Straight Lines

3.3.2Parallel & Perpendicular Lines

4Year 3 - Statistics

4.1Statistics

4.1.1Statistics

4.1.2Pictograms & Bar Charts

5Year 4 - Number

6Year 4 - Measurement

6.1Units

6.1.1Metric Measurements

6.1.2Adding & Subtracting Money

6.2Time

6.2.1Converting Units of Time

6.2.2Changing the 12-Hour Clock to the 24-Hour Clock

6.2.3Changing the 24-hour Clock to the 12-Hour Clock

7Year 4 - Geometry

7.1Properties of Shapes

7.1.1Symmetry

7.1.2Types of Triangle

7.1.3Quadrilaterals

7.2Perimeter & Area

7.2.1Perimeter

7.2.2Area

7.3Position & Direction

7.3.1Coordinates

7.3.2Translation

8Year 4 - Statistics

8.1Statistics

8.1.1Types of Data

8.1.2Pictograms & Bar Charts

8.1.3Line Graphs

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