Transcript Slide 1

Decomposition
I am learning a standard method of doing a subtraction problem on
paper when the numbers are too difficult to find the answer
mentally.
In your head
Numbers are set out directly
below each other.
e.g. 243
- 165
There are 536 people at a sports game. 358 people are wearing
black. How many people are not wearing black? 536 - 358 = ?
Step 1: For our first example let’s expand each number and write
them underneath each other.
400 120
20 16
500 + 30 + 6
Step 2: There are not enough ones to subtract 8. This means
we must break up a ten and add it to the six.
Step 3: Now subtract the ones. (16 – 8 = 8).
- 300 + 50 + 8
100 + 70 + 8
Step 4: There are not enough tens to subtract 50. This means we must
break up a hundred and add it to the 20.
Step 5: Now subtract the tens. (120 – 50 = 70)
Step 6: Now subtract the hundreds. (400 – 300 = 100)
Step 7: Add the bottom line together to get your answer.
100 + 70 + 8 = 178
536 – 358 = 178
There were 732 people in the hall. 309 people left through Door One.
How many people did not leave through Door One? 732 - 309 = ?
Step 1: Now let’s try the same algorithm without expanding the
number.
212
732
-309
423
Step 2: There are not enough ones to subtract 9. This means we
must break up a ten and add it to the two.
Step 3: Now subtract the ones. (12 – 9 = 3)
Step 4: Subtract the tens. (2 tens – 0 tens = 2 tens)
Notice how we just record a 2 in the tens column. Not a 20.
Step 5: Subtract the hundreds. (7 hundreds – 3 hundreds = 4 hundreds)
Notice how we just record a 4 in the hundreds column. Not 400.
The bottom line is the answer. 423
732 – 309 = 423
Now try these questions on your own. You will need to use pen and paper
to record your algorithm.
1) 642 - 238 = ?
404
2) 537 - 188 = ?
349
3) 723 - 144 = ?
579
4) 365 - 206 = ?
159
5) 552 – 206 = ?
346
Click again when you have
completed each question and
the answer will be displayed.
Need some more practise? Try Pg60 of Teacher Tools Addition and
Subtraction Book Numeracy resources