2.1.1

Method A - Decomposition Method

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Complete the interactive activities in the Seneca app to work towards your CPD certificate.

Decomposition Method: 753 − 491

Step 1

Step 1

  • Write out the subtrahend under the minuend.
  • Line up the digits so that the hundreds, tens and ones digits of each number are in the same columns.
Step 2

Step 2

  • Start at the right. Subtract 1 from 3.
  • Write the difference at the bottom, in the ones column.
Step 3

Step 3

  • Now we want to ‘subtract 9 from 5'. Note that because we’re in the tens column now, we’re actually subtracting 90 from 50.
  • To do this without using negatives, we exchange the 700 (i.e. we see 700 as 600 + 100). We do this by crossing out the 7 in the hundreds column and replacing it with a 6.
Step 4

Step 4

  • Then we write a small 1 to the left of the 5 in the tens column, giving us 15 tens.
  • What we’ve done is add one of our hundreds to the 50, meaning we can now to subtract 90 from 150.
Step 5

Step 5

  • Now, we subtract 9 from 15 which gives a difference of 6 for the tens column (i.e. 150 − 90= 60).
Step 6

Step 6

  • Finally, we subtract the digits in the hundreds column.
  • We’ve used one of our hundreds already, so we have 6 left. We do 6 − 4 and write a 2 at the bottom. (This actually means 600 − 400 = 200.)
Breaking down the maths

Breaking down the maths

  • What we effectively did here was rewrite 753 as 600 + 150 + 3.
Jump to other topics
1

Course Overview

1.1

Course Structure & Commonly Asked Questions

1.1.1Introduction to the Course & Common Questions
2

Subtraction: KS2/3

2.1

Subtraction Methods

2.1.1Method A - Decomposition Method2.1.2Method B - Equal Addition Method2.1.3Method C - Expanded Form Method2.1.4Method D - Partitioning Method2.1.5Method E - Counting-Up Method2.1.6Method F - Constant Difference Method2.1.7Method G - Partial Differences Method2.1.8Method H - Complementary Method2.1.9Method I - Nines Complement Method
3

Expanding Double Brackets: KS3/4

3.1

Expansion Methods

3.1.1Method A - Grid Method3.1.2Method B - Each by Each3.1.3Method C - Distributive Law3.1.4Method D - Inspection3.1.5Method E - Column Method3.1.6Method F - Vertical & Crosswise

Practice questions on Method A - Decomposition Method

Can you answer these? Complete these activities in the Seneca app to progress towards your certificate.

  1. 1
    Which digit do students need to exchange (i.e. cross out and replace) in the top line of this subtraction?Multiple choice
  2. 2
    Which digit do students add a small 1 to, when using the decomposition method to perform this calculation?Multiple choice
  3. 3
    What calculation can a student do to immediately check that 9610 is the correct answer to 9875 − 265?Multiple choice
Answer all questions on Method A - Decomposition Method

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Introduction to the Course & Common Questions

Method B - Equal Addition Method