Mental Math Strand B – Number Sense Grade 2
Jan 03, 2016
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
Front End Addition with Regrouping
74 + 815 + 1674 + 1953 + 9 45 + 763 + 727 + 834 + 672 + 1914 + 27
Front End Addition with Regrouping
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
Subtraction – Near Doubles
13 – 617 – 8 11 – 5 7 – 315 – 717 – 819 – 99 – 4
21 – 105 – 2
Subtraction – Near Doubles
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.
Using Subtraction Facts for 10s
90 – 1030 – 2060 – 3020 – 1070 – 6080 – 3050 – 2040 – 3020 – 1080 - 50
Using Subtraction Facts for 10s
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