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Mental Subtraction
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Foundational skills
Subtraction within 10
\[ 9 – 4 = 5 \]
Subtract single digit numbers.
Subtracting from 10
\[ 10 – 7 = 3 \]
Find complements to 10.
Halves within 20
\[ 14 – 7 = 7 \]
Subtract half of an even number.
Near halves within 20
\[ 15 – 7 = 8 \]
Subtract a number close to half.
Subtracting within 20
Single digit from teen (no bridge)
\[ 17 – 4 = 13 \]
Answer stays above 10.
Single digit from teen (bridging 10)
\[ 15 – 7 = 8 \]
Answer is less than 10.
Teen from teen
\[ 18 – 13 = 5 \]
Subtract one teen from another.
Subtracting from 100
Multiple of 10 from 100
\[ 100 – 30 = 70 \]
Subtract 10, 20… from 100.
Two-digit number from 100
\[ 100 – 37 = 63 \]
Subtract any two-digit number.
Subtracting with two-digit numbers
Single from two-digit (no bridge)
\[ 47 – 3 = 44 \]
No change to tens digit.
Single from two-digit (bridging)
\[ 43 – 7 = 36 \]
Cross into previous ten.
Multiple of 10 from two-digit
\[ 67 – 30 = 37 \]
Subtract tens.
Two-digit from two-digit (no bridge)
\[ 58 – 23 = 35 \]
Subtract tens and units separately.
Two-digit from two-digit (bridging)
\[ 54 – 27 = 27 \]
Requires bridging tens.
Subtracting multiples of 10
Two multiples of 10 within 100
\[ 70 – 40 = 30 \]
Subtract tens within 100.
Two multiples of 10 (bridging 100)
\[ 130 – 50 = 80 \]
Cross 100 boundary.
Multiple of 10 from 3-digit (no bridge)
\[ 270 – 40 = 230 \]
Subtract tens without crossing 100.
Multiple of 10 from 3-digit (bridging)
\[ 230 – 50 = 180 \]
Subtract tens crossing a hundred.
Compensation strategies
Subtracting 9 from teen
\[ 16 – 9 = 7 \]
Subtract 10, add 1.
Subtracting 8 from teen
\[ 15 – 8 = 7 \]
Subtract 10, add 2.
Subtracting 9 from two-digit
\[ 53 – 9 = 44 \]
Subtract 10, add 1.
Subtracting 8 from two-digit
\[ 45 – 8 = 37 \]
Subtract 10, add 2.
Subtracting 19
\[ 54 – 19 = 35 \]
Subtract 20, add 1.
Subtracting 29
\[ 67 – 29 = 38 \]
Subtract 30, add 1.
Subtracting 39
\[ 78 – 39 = 39 \]
Subtract 40, add 1.
Subtracting 18
\[ 56 – 18 = 38 \]
Subtract 20, add 2.
Subtracting 28
\[ 65 – 28 = 37 \]
Subtract 30, add 2.
Subtracting 38
\[ 74 – 38 = 36 \]
Subtract 40, add 2.
Subtracting 99
\[ 245 – 99 = 146 \]
Subtract 100, add 1.
Subtracting 98
\[ 356 – 98 = 258 \]
Subtract 100, add 2.
Subtracting with three-digit numbers
Two multiples of 100
\[ 700 – 300 = 400 \]
Subtract hundreds.
Two three-digit multiples of 10
\[ 580 – 230 = 350 \]
Subtract tens and hundreds.
Single digit from three-digit (no bridging)
\[ 567 – 4 = 563 \]
Subtract units only.
Single digit from three-digit (bridging tens)
\[ 543 – 7 = 536 \]
Bridge tens only.
Single digit from three-digit (bridge tens & hundreds)
\[ 503 – 7 = 496 \]
Bridge tens and hundreds.
Timer (Optional)
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