2.1.1

Method A - Decomposition Method

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Decomposition Method: 753 − 491

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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.
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Step 2

  • Start at the right. Subtract 1 from 3.
  • Write the difference at the bottom, in the ones column.
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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.
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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.
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Step 5

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

  • What we effectively did here was rewrite 753 as 600 + 150 + 3.

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