Mental Subtraction Strategies
This mental math lesson explains 6 different strategies you can use for mentally subtracting 2-digit numbers, meant for 2nd or 3rd grade. It contains the instruction, examples, lots of exercises, and word problems for students to complete.
| Strategy 1: Subtract in two parts | |||||||||||||||||||||||||||||||||||||||||||||
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1. Subtract the elevated number in parts first subtract to the previous whole ten; then the rest.
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2. First subtract the balls that are not in the ten-groups.
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| Strategy 2: Use known subtraction facts |
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Since 14 − 6 = 8, we know that the answer to 74 −
6 will end in 8, Since 15 − 8 = 7, we know that
the answer to 55 − 8 will end in 7, |
3. Subtract. The first problem in each box is a “helping problem” for the others.
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a. 14 − 9 = _______ 24 − 9 = _______ 44 − 9 = _______ |
b. 17 − 8 = _______ 27 − 8 = _______ 37 − 8 = _______ |
c. 12 − 9 = _______ 52 − 9 = _______ 32 − 9 = _______ |
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d. 15 − 9 = _______ 65 − 9 = _______ 45 − 9 = _______ |
e. 13 − 8 = _______ 33 − 8 = _______ 93 − 8 = _______ |
f. 16 − 8 = _______ 86 − 8 = _______ 36 − 8 = _______ |
4.
a. Amy has $32. She
bought a comic book for $7.
How much does she have now?
b. Peter had
$29. A toy train he wants costs $39.
Mom paid him $5 for working. How
much more
does Peter now need to buy the
train?
c. A flower
shop has 55 roses. Eight of them are white,
and the rest are red. How many are red?
| Strategy 3: Subtract in parts: tens and ones | |||||||||||||||||||||||||||||||||||||||||
| Break the number being subtracted into its tens and ones. Subtract in parts. | |||||||||||||||||||||||||||||||||||||||||
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5. Subtract in parts: Break the second number into its tens and ones.
= __________ |
= __________ |
= __________ |
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d. 69 − 19 |
e. 67 − 36
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f. 64 − 33 |
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| Strategy 4: Add. | |
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You can “add backwards”. This works well if the two numbers are close to each other. Instead of subtracting, think how much you need to add to the number being subtracted (the subtrahend) in order to get the number you are subtracting from (the minuend). |
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71 − 67 = ?? Think: 67 + ____ = 71 |
558 − 556 = ?? Think: 556 + ____ = 558 |
6. Subtract.
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a. 78 − 75 = _______ 61 − 58 = _______ |
b. 112 − 108 = _______ 692 − 688 = _______ |
c. 505 − 499 = _______ 1000 − 994 = _______ |
7.
You had $50. You purchased two bouquets of roses for $13 each.
How much do you have left after the purchase?
8.
What if you bought three bouquets of roses for $13 each with your $50?
How much would you have left after the purchase?
9. Fifteen
children were playing on the playground. Seven of them left.
Then, ten more children came. How many are playing on the
playground now?
10. A lion chased
an antelope for 400 feet, then another 200 feet, and
lastly 200 feet more. Then the lion pounced on the antelope.
What was the total number of feet that the lion
chased the antelope?
| Strategy 5: Add up to find the difference. | |||||||||||||||||||||
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To find the difference, start at the smaller number, and add up until you get to the bigger number. You can first complete the next ten, then add whole tens, then add ones again.
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11. Add up to find the differences, or use some other strategy such as subtracting in parts.
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c. 56 − 28 = _______ 55 − 24 = _______ |
d. 72 − 18 = _______ 82 − 46 = _______ |
e. 54 − 37 = _______ 91 − 57 = _______ |
f. 74 − 55 = _______ 63 − 34 = _______ |
| Strategy 6: Subtract an easy number that is close, and then correct the answer. |
74 − 39 = ? First subtract
74 − 40 = 34, since 40 is |
81 − 57 = ? First subtract 81 − 60 = 21, since
60 is |
12. Subtract mentally.
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a. 34 − 18 = ______ 42 − 29 = ______ |
b. 65 − 27 = ______ 55 − 38 = ______ |
c. 97 − 49 = ______ 62 − 19 = ______ |
d. 65 − 29 = ______ 83 − 38 = ______ |
13. A subtraction challenge!
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a. 101 − _____ = 92 |
b. ________ − 60 = 7 |
c. ________ − 132 = 40 |
14. Another challenge!
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100 |
______ | _____ | _____ | _____ | _____ | _____ | |||||||||||||||||||
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This lesson is taken from Maria Miller's book Math Mammoth Add & Subtract 3, and posted at www.HomeschoolMath.net with permission from the author. Copyright © Maria Miller.
Math Mammoth Add & Subtract 3
A self-teaching worktext for 2nd-3rd grade that covers mental addition & subtraction with three-digit numbers, regrouping with three-digit numbers (carrying and borrowing), rounding, estimating, and word problems.
Download ($7.10). Also available as a printed copy.
