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The ideas in this addition lesson are taken from Add & Subtract 3 ebook. Only a few examples of each problem type are shown; you should make more problems of each kind for the student. Subtracting 2-digit numbers mentally
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| Strategy 1: Subtract in two parts | ||||||||||||||||||
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57 − 25 = ? Break the 25 into tens and ones. First subtract 20, then subtract 5.
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1. Subtract mentally by breaking the second number into tens and ones.
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2. How can you compare these expressions without actually subtracting?
Write < or > .
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60 − 28 |
60 − 25 |
90 − 25 |
90 | |||
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70 − 24 |
70 − 36 |
97 − 32 |
90 − 32 |
| Strategy 2: Add up to find the difference. | ||||||||||
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Subtraction gives you the difference between two numbers. To find the difference, you can start at the smaller number, and add up till you get to the bigger number. When adding up, first complete the ten, then add whole tens, then ones again.
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3. Add up to find the difference.
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56 − 28 =
24 − 55 = |
72 − 18 =
82 − 46 = |
74 − 55 =
63 − 34 = |
4. Find missing addends. The same method works here. Think: first add
up to whole ten, then see how much more you need.
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a. 37 + __ = 70 28 + __ = 90 54 + __ = 80 |
d. 36 + __ = 64 65 + __ = 80 65 + __ = 83 |
| Strategy 3: Subtract an easy number that is close, then correct the answer. |
This strategy works well if the number you're subtracting is a little less than a whole ten. You instead subtract the whole tens, and then add back however much the "error" was.
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5. Subtract mentally using the above strategy.
| 34 − 18 =
42 − 29 = 76 − 59 = |
65 − 27 =
55 − 38 = 94 − 48 = |
6. Subtract mentally, using any of the methods - or one of your own!
| 66 − 38 =
93 − 57 = 27 − 41 = 62 − 35 = 88 − 49 = |
55 − 46 =
74 − 28 = 48 − 13 = 74 − 53 = 91 − 59 = |
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