Download N1/E3.2

N1/E3.2
Worksheet Two
”Subtraction – decomposition”
You may have been taught this subtraction method at school. It involves
splitting the sum into hundreds, tens and units.
Have a look at this sum:
192 - 44
Before you begin, try to get a rough idea of what the answer is going to
be:
192 can be rounded up to 200, &
44 can be rounded up to 50.
So…
192 - 44 is approximately 200 – 50 = 150
So before we start, we know that the final answer will be about 150.
(this is called an ESTIMATE.
It allows us to check our final answer is reasonable.)
Now do the sum - remembering to start with the units first.
Finding Ten
If one of the columns has a smaller number on top, the bottom number
cannot be taken away e.g. in the example below, 2- 4 cannot be done!
Start with the UNITS column first.
2 is smaller than 4, so we need to add ten from the
9 lots of ten.
So 9 becomes 8 and 2 becomes 12
12 – 4 = 8
Put 8 in the UNITS column.
Move to the TENS column.
8-4=4
Put 4 in the TENS column.
Move to the HUNDREDS column.
1+0=1
Put 1 in the HUNDREDS column
So 192 - 44 = 148
Checking your answer
Now we must check the answer makes sense.
Before we began, we ESTIMATED the answer would be about 150
Our final answer was 148
So the answer makes sense!
Exercise
1)
378
- 243
2)
499
- 345
3)
764
- 406
4)
657
33
5)
6)
963
- 200
7)
244
27
8)
553
- 107
9)
144
- 115
10)
396
- 292
754
46
Borrowing because of a zero
This is the same as if one of the columns has a smaller number on top, you
borrow from the number to its left.
205 - 33
Start with the UNITS column first.
5–3 =2
Put 2 in the UNITS column.
Move to the TENS column
0 is smaller than 5, so we must borrow from
the 2 in the HUNDREDS column
So 0 becomes 10 & 2 becomes 1.
10 – 5 = 5
Put 5 in the TENS column
Move to the HUNDREDS column.
1+0=1
Put 1 in the HUNDREDS column
So 205 - 53 = 152
Remember:


If the top number is smaller, borrow from the left.
If a zero appears in the top number you will probably need to
borrow.
Exercise
1)
602
- 363
2)
803
- 444
3)
501
- 222
4)
904
- 137
5)
6)
208
65
7)
600
- 123
8)
900
- 497
9)
409
36
10)
703
- 567
500
- 341
Addition and subtraction are opposites.
You can check the answer to a subtraction sum by turning the numbers
around and adding them up. Have a look below.
Here you've done the sum 10 - 25 and got the answer 15.
To check the answer, turn the sum around to 15 + 10 and see if you get
25.
Exercise Four – Part Three
a) Check ALL your answers to the questions from exercise two, we’ve
done the 1st one for you!
1)
602
- 363
239
2)
803
- 444
3)
501
- 222
As we get 602
here we KNOW
that our answer
for the
subtraction was
correct!
239
+ 363
602
+ 444
Don’t be a lazy
toad, write the
rest of them out
yourself!!!
+ 222