Developing an understanding in number
6. Strategies for subtraction
When teaching basic facts, it is essential that addition facts are known before
introducing (related) subtraction facts.
For example, a child asked to solve 7 – 2 = ? will find the task much easier if
he/she knows 5 + 2 = 7.
| a) |
|
Count-on subtraction |
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This strategy is useful when the difference between the minuend and the
subtrahend is no greater than 3. |
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For example: |
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|
| 8 |
- |
5 |
= |
3 |
| minuend |
|
subtrahend |
|
difference
(remainder) |
|
| |
|
Start with the smaller number and count the number of steps required to reach
the larger number (don’t count the smaller one). |
| |
|
| |
9 |
- |
6 |
= |
? |
| |
6, 7, 8, 9 |
|
|
|
| |
9 |
- |
6 |
= |
3 |
| (extension) |
68 |
- |
66 |
= |
? |
| |
66, 67, 68 |
|
|
|
| |
68 |
- |
66 |
= |
2 |
|
| |
| b) |
|
Subtract zero |
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|
The difference is the same as the minuend. |
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|
|
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| c) |
|
Subtract one |
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The remainder is the number before the minuend. |
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|
|
| |
| d) |
|
Subtract multiples of ten (and one hundred) |
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|
The ones digit from the minuend remains the same; only the tens (or hundreds)
digits change. |
| |
|
| 75 |
- |
30 |
= |
45 |
|
325 |
- |
100 |
= |
225 |
| 70 |
- |
30 |
= |
40 |
|
300 |
- |
100 |
= |
200 |
|
| |
| e) |
|
Counting down |
| |
|
This strategy is useful when the number to be subtracted is small, for example
1, 2, or 3. |
| |
|
| |
14 |
- |
3 |
= |
? |
| |
Begin at 14, count down 3. |
| |
14, 13, 12, 11 |
| |
14 |
- |
2 |
= |
11 |
| (extension) |
88 |
- |
3 |
= |
? |
| |
88, 87, 86 |
| |
88 |
- |
2 |
= |
86 |
|
| |
| f) |
|
Doubles subtraction |
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|
Known doubles facts can be used to solve a subtraction fact. |
| |
|
| |
10 |
- |
5 |
= |
? |
|
| |
5 |
+ |
5 |
= |
10 |
(known fact), so... |
| |
10 |
- |
5 |
= |
5 |
|
| (extension) |
50 |
- |
25 |
= |
? |
|
| |
25 |
+ |
25 |
= |
50 |
(known fact), so... |
| |
50 |
- |
25 |
= |
25 |
|
|
| |
| g)) |
|
Complementary addition |
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|
When using this strategy, the subtraction question is converted to an addition
question. |
| |
|
| |
18 |
- |
12 |
= |
? |
|
| If |
12 |
+ |
6 |
= |
18 |
is a known fact, it's
possible to say... |
| |
18 |
- |
12 |
= |
6 |
|
|
| |
|
2D shapes, 3D objects, Nets
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