DECOMPOSITION - SUBTRACTION METHOD


Example:    8 4 3
                 - 2 6 6

STEP 1  

  • Take 6 ones from 3 ones, can't do

  • Regroup the 4 tens by trading so there are 3 tens left in the tens column and 10 ones have been added to the original 3 to give  13 ones

  • 6 ones are subtracted from 13 ones

  • 7 is placed on the answer line in the ones place


STEP 2

  • 6 tens can't be subtracted from 3 tens

  • Regroup the 8 hundreds by trading so there are 7 hundreds in the hundreds column and the 10 tens have been added to the 3 tens to give 13 tens in the tens column

  • 6 tens are subtracted from 13 tens

  • 7 is placed in the answer line in the tens place


STEP 3

  • Subtract 2 hundreds from the 7 hundreds

  •  5 is placed on the answer line in the hundreds place

 


 

 

A few more examples of the decomposition method

Extension idea:
Many children love the challenge of doing subtractions mentally; indeed, many strategies are often discussed in class. Eg

843 - 266
I can first take away the 2 hundred
Now I have 643 - 66
I can also take away 6 tens from 64 tens
Now I have 583 - 6
I can take the 6 ones from 83 ones
Now I have 577
 

843 - 266
I can first take away 243
This leaves 600
I now need to take away another 23
Answer 577

How many other ways can you find?