mental math strand b – number sense grade 2. addition – plus 1 facts 7+1 or 1+7 is asking for...

Mental Math

Strand B – Number Sense

Grade 2

Addition – Plus 1 Facts

• 7+1 or 1+7 is asking for the number after 7. One more than 7 is 8. 7+1=8, 1+7=8

• 4 +1 is asking for the number after 4. One more than 4 is 5. 4+1=5

Plus 1 Facts

2+1=8+1=6+1=9+1=4+1=8+1=5+1=3+1=1+1=7+1=

Plus 1 Facts

8+1=1+4=1+9=6+1=1+8=2+1=7+1=1+5=1+1=3+1=

Addition – Plus 2 Facts

6+2 is skip counting by 2 after 6 or asking for the next even number after 6 to get 8. 6+2=8

1+2 is skip counting by 2 after 1.

1+2=3

Plus 2 Facts

1+2=3+2=7+2=2+2=5+2=9+2=4+2=0+2=8+2=6+2=

Plus 2 Facts

1+2=3+2=2+7=2+2=2+5=9+2=2+6=2+4=0+2=2+8=

Addition-Doubles Facts

1+1: The number of wheels on two unicycles2+2: The number of wheels on two bicycles3+3: The number of wheels on two tricycles4+4: The number of wheels on two cars5+5: The number of fingers on two hands6+6: The number of eggs in an egg carton7+7 The number of days in two weeks8+8 the number of crayons in a two-row box9+9: the number of tires on an 18-wheeler

Double Facts

4+4=1+1=5+5=0+0=6+6=7+7=9+9=8+8=3+3=2+2=

Double Facts

2+2=9+9=0+0=5+5=7+7=3+3=4+4=1+1=6+6=8+8=

Addition- Near Doubles

• This strategy is also called the “doubles plus one” facts.

• Double the smaller number and add one more.

6+7 is (6+6)+1

6+5 is double 5 and add 1 or 10+1=11

Near Doubles

8+9=8+7=2+3=3+4=2+1=5+6=4+5=6+5=0+1=6+7=

Near Doubles

5+4=2+1=3+4=2+3=7+8=8+9=5+6=4+3=7+6=0+1=

Addition- Doubles Plus 2

This strategy is also called 2-apart facts

3+5: Double the smaller number plus 2

3+5 = 6 (double 3) +2 =8

4+6= 8 (double 4) +2 =10

Doubles Plus 2

4+6=5+7=6+8=7+5=3+5=7+9=5+3=6+4=9+7=0+2=

Doubles Plus 2

1+2=5+3=6+4=5+7=8+6=7+9=3+5=2+0=2+1=9+7=

Addition – Plus or Minus 0 “No Change”

• Adding does not always make numbers bigger. 5+0=5. Adding zero involved no change.

• Subtracting does not always make numbers smaller. 4-0=4. Subtracting zero involved no change.

Plus or Minus 0. “No Change”

5+0=2+0=0+6=4+0=0+9=

10+0=6-0=1-0=8-0=7-0=

Plus or Minus 0“No Change”

3-0=8-0=

10+0=0+6=4-0=1-0=0+9=2+0=7-0=6+0=

Addition – Making 10 Using Ten Frames

• Visualize 6 counters on this ten frame. This leaves 4 boxes on the ten-frame empty for a total of 6+4=10

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Make 10 Using a Ten Frame

7+3=7+3=10

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Make 10 Using a Ten Frame

4+6=4+6=10

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Make 10 Using a Ten frame

8+2=8+2=10

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Addition –Make 10

• 9+3: Think, 9+1 (from the 3) is 10,

and 10 +2 (the other part of the 3) is 12

or 9+3= (9+1)+2=10+2=12

• 7+4: Think 7 and 3 (from the 4) is 10,

and 10+1 (the other part of the 4) is 11

or 7+4= (7+3) +1=10+1=11

*Students may wish to use a ten frame to solve.

Make 10

9+4=

5+9=

8+5=

9+6=

6+8=

3+9=

8+4=

8+6=

7+9=

8+3=

Make 10

7+5=

8+7=

7+6=

3+7=

7+7=

9+7=

7+4=

5+7=

6+7=

7+8=

Addition – Counting On

• 2+6: Start with the larger number and COUNT ON the amounts of the

smaller number (six, SEVEN, EIGHT).

• 2+45: Start with 45 and COUNT ON forty-five, FORTY-SIX, FORTY-SEVEN).

Counting On

7+3=

3+6=

8+2=

1+5=

1+9=

3+9=

1+8=

5+2=

7+1=

5+3=

Counting On

1+6=

4+2=

2+7=

3+8=

2+6=

43+3=

2+51=

2+47=

26+3=

3+45=

Addition – 10 and “Some More”

• 10 + 4, think 10 and 4 is 14

• 10 + 8, think 10 and 8 is 18

• 28 + 4, think 28 + 2 is 30 and 30 + 2 is 32

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10 and “Some More”

10 + 210 + 510 + 13 +1010 + 68 + 1010 + 510 + 97 + 1010 + 4

10 and “Some More”

27 + 615 + 957 + 568 + 737 + 887 + 948 + 49 + 7769 + 68 + 16

Addition – Using Addition Facts for 10’s (Doubles)

• If you know that 6 + 6 = 12, then you know that 60 + 60 =120

• If you know that 5 + 5 = 10, then you know that 50 + 50 = 100

• If you know that 4 + 4 = 8, then you know that 40 + 40 = 80

Using Addition Facts for 10’s(Doubles)

40 + 4050 + 5080 + 8020 + 2070 + 7010 + 1030 + 3090 + 9060 + 60

Using Addition Facts for 10’s(Doubles)

90 + 9070 + 7030 + 3010 + 1080 + 8050 + 5020 + 2040 + 4060 + 60

Using Addition Facts for 10’s (Near Doubles)

• If you know that 2 + 3 = 5, then you know that 20 + 30 is (20 + 20) + 10 = 50.

Say “ 20 + 30 is double 20 and add 10; 40 + 10 is 50.”

• 30 + 40 is (30 + 30) + 10 = 70

Using Addition Facts for 10’s(Near Doubles)

30 + 4010 + 2050 + 4070 + 80 60 + 7080 + 9050 + 6020 + 3040 + 5090 + 80

Using Addition Facts for 10’s(Near Doubles)

80 + 9050 + 4090 + 8070 + 6040 + 5020 + 1040 + 3030 + 2060 + 5070 + 80

Using Addition Facts for 10’s(Doubles plus 2)

• If you know that 3 + 5 = 8, then you know that 30 + 50 is (30 + 30) + 20 = 80

• 40 + 60 is (40 + 40) + 20 = 100

Using Addition Facts for 10’s( Doubles Plus 2)

40 + 6070 + 9050 + 3060 + 4050 + 7090 + 7060 + 8070 +5020 + 4010 + 30

Using Addition Facts for 10’s( Doubles Plus 2)

50 + 7080 + 6070 + 9070 + 5040 + 6030 + 5090 + 7060 + 4030 + 1020 + 40

Quick Addition – No RegroupingFront End Addition

• 56 + 23, add the highest place values first

5 + 2 = 7, then add add the sum of the next place value, 6 + 3= 9. 56 + 23 = 79

• 71 + 12 = 7 + 1 =8, 1 +2 = 3 = 83

Front End Addition

71 + 1237 + 5163 + 3315 + 6244 + 5366 + 2356 + 2282 + 1127 + 1273 + 24

Front End Addition

62 + 3544 + 2392 + 1732 + 5681 + 1928 + 7056 + 3222 + 7414 + 6351 + 37

Front End Addition with Regrouping

• To solve 46 + 38 you can join the tens (7 tens) and the ones (14 ones), and rename the sum.

46 + 38 = 70 + 14 =84


74 + 815 + 1674 + 1953 + 9 45 + 763 + 727 + 834 + 672 + 1914 + 27


75 + 716 + 1576 + 1854 + 944 + 846 + 3819 + 1421 + 963 + 2834 + 7

Addition _ Finding Compatibles

• This strategy involves looking for pairs if numbers that add to powers of 10 (10 or 100)

3 + 8 + 7, think 3 + 7 is 10 plus 8 is 10 + 8 = 18

• 6 + 9 + 4, think 6 + 4 is 10 plus 9 is 10 + 9 =19

Finding Compatibles

6 + 9 + 43 + 8 + 72 + 3 + 84 + 6 + 21 + 9 + 55 + 6 + 52 + 8 + 76 + 5 + 46 + 7 + 34 + 9 + 1

Finding Compatibles

5 + 6 + 51 + 9 + 54 + 6 + 28 + 3 + 24 + 9 + 66 + 7 + 49 + 1 + 37 + 5 + 38 + 4 + 25 + 5 + 1

Addition - Compensation

• This strategy involves changing one number to a ten; carrying out the addition and adjusting the answer to compensate for the original change. This strategy is useful when one of the numbers ends in 8 or 9.

7 + 9, think 7 + 10 is 17, but I added one too many, so I subtract one from 17 to get 16 or

7 + 9 = (7 + 10) –1 =16

Compensation

2 + 93 + 99 + 45 + 89 + 58 + 79 + 68 + 37 + 87 + 9

Compensation

8 + 78 + 36 + 98 + 75 + 98 + 54 + 99 + 39 + 29 + 7

Subtraction- Doubles

• This strategy uses the addition double facts to help find the answers to related subtraction combinations.

12 – 6, think 6 plus what makes 12? 6 + 6 =12, so 12 – 6 = 6

If you know 8 – 4 = 4, then 80 – 40 = 40

Subtraction – Doubles

10 – 516 – 86 – 38 – 4

14 – 718 – 912 – 64 – 2

20 – 102 - 1

Subtraction – Doubles

20 – 1040 – 20160 – 80140 – 7020 – 10120 – 60 100 – 50180 – 9060 – 3080 – 40

Subtraction – Near Doubles

• This strategy also uses the addition double facts to help find the answer.

9 – 4, think: 4 + 4 = 8, 4 + 5 = 9, so 9 – 4 = 5

If you know that 9 – 4 = 5,

then for 90 – 40, think 40 + 40 = 80; 40 + 50 = 90,

so 90 – 40 = 50


13 – 617 – 8 11 – 5 7 – 315 – 717 – 819 – 99 – 4

21 – 105 – 2


70 – 30110 – 5050 – 20150 – 70130 – 60170 – 8090 – 40190 – 9030 – 1070 - 30

Subtraction – “Think Addition”

• Use your knowledge of addition facts to find the answers to subtraction equations.

9 – 4, think: 4 + what = 9?

80 – 20, think: 20 + what = 80?

Subtraction – Think Addition

10 – 4 =___ ( 4 plus what equals 10?)10 – 48 – 59 – 3 8 – 210 – 36 – 4

10 – 67 – 38 – 4

Subtraction – Think Addition

80 – 2090 – 30

100 – 3060 – 4080 – 5070 – 40

100 – 4060 – 3090 – 5070 - 20

Using Subtraction Facts for 10s

• If we know that 8 - 3 = 5, then for 80 – 30, think: 8 tens subtract 3 tens is 5 tens or 50

• 90 – 10, think: 9 tens subtract 1 ten is 8 tens or 80.


90 – 1030 – 2060 – 3020 – 1070 – 6080 – 3050 – 2040 – 3020 – 1080 - 50


80 – 2070 – 5020 – 1060 – 4030 – 1090 – 70

50 - 2080 – 3060 – 30 90 - 40

Addition & SubtractionFront End Estimation

• This strategy involves combining only the values in the highest place value to get a “ball-park figure”

43 + 54, think 40 + 50 is 90

92 – 53, think: 90 - 50 is 40

Addition Front End Estimation

62 + 3154 + 3313 + 8244 + 2371 + 1434 + 4273 + 1221 + 4312 + 5122 + 46

SubtractionFront End Estimation

92 – 5393 – 6272 – 3343 – 1264 – 2354 – 2132 – 2381 – 5491 – 4284 - 61

Addition & SubtractionRounding

• This strategy involves rounding each number to the highest, or highest two place values and adding the rounded numbers.

To estimate 27 + 31, think: 27 rounds to 30 and 31 rounds to 30, so 30 + 30 is 60.

To estimate 87 – 32, think: 87 rounds to 90 and 32 rounds to 30, so 90 – 30 is 60.

Addition -Rounding

48 + 2361 + 4897 + 1228 + 5734 + 5918 + 2214 + 3241 + 3427 + 3145 + 63

Subtraction - Rounding

57 –1436 – 2268 – 3484 – 943 – 899 – 4782 – 5954 – 1893 –1287 – 32

mental math strand b – number sense grade 2. addition – plus 1 facts 7+1 or 1+7 is asking for...

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