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Lesson · Grade 5 · Mathematics

Friendly Numbers: Adding by Compensating

  • Grade 5
  • Mathematics
  • 1 min 15 s
  • British Columbia
  • BC5-MA-12

To work out 3998 + 2475, the video changes 3998 into the friendly number 4000, which is just 2 more, and finds 4000 + 2475 = 6475 by adding the thousands. Since 2 too many were added, it takes 2 away to get 6473, and ends on the recap "Make one number friendly, then undo the change".

Concept: Friendly numbers: adding and subtracting by compensating

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In one sentence

The question this lesson answers

What is 3998 plus 2475?

3998 is very close to 4000.

Transcript

What the lesson says

What is 3998 plus 2475?

3998 is very close to 4000.

It needs just 2 more to make 4000.

4000 is a friendly number, because it is easy to add.

So add 4000 instead of 3998.

4 thousands plus 2 thousands make 6 thousands.

So 4000 plus 2475 is 6475.

But we added 2 too many.

So take those 2 away.

6475 minus 2 is 6473.

So 3998 plus 2475 is 6473.

Adding 2 and then taking 2 away makes no change.

This way of adding is called compensating.

Make one number friendly, then undo the change in the answer.

Curriculum

Where this fits

British Columbia, Grade 5

Mathematics, Number: addition and subtraction of whole numbers to 1 000 000BC5-MA-12

Previous: Subtracting Big Numbers: Regrouping Across ZerosNext: Multiplying with an area model: 23 × 14

The outcomes and strands follow the British Columbia curriculum.

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