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May 2012

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The ideas in this addition lesson are taken from Add & Subtract 2-B ebook. Only a few examples of each problem type are shown; you should make more problems of each kind for the student.

Adding 2-digit numbers mentally
Free lesson plan from Homeschool Math.net

There are several strategies that you can use when adding two 2-digit numbers mentally.

Strategy 1: Break the second addend in parts

45 + 27 = ? Break the 27 into tens and ones. First add 20, then add 7:

45 +	27			=
45 +	20	+	7	=
65		+	7	= 72.

1. Add by breaking the second addend into tens and ones.

52 +	26			=
52 +	20	+	6	=
		+	6	=

38 +	35		=
38 +		+	=
		+	=

25 +	57		=
25 +		+	=
		+	=

62 +	19		=
62 +		+	=
		+	=

2. Add mentally.

34 + 35 =

47 + 12 =

33 + 56 =

27 + 26 =

19 + 18 =

13 + 17 =

34 + 18 =

45 + 35 =

Strategy 2: Add the tens and ones separately

45 + 27 = ? Add tens. Add ones. Add the two sums:

45 + 27		=
(40 + 20) +	(5 + 7)	=
60 +	12	= 72.

3. Add by adding tens and ones separately.

36 + 22		=
(30 + 20) +	(6 + 2)	=
+		=

72 + 18		=
(70 + 10) +	(2 + 8)	=
+		=

54 + 37		=
(50 + 30) +	(4 + 7)	=
+		=

24 + 55		=
(__ + __) +	(_ + _)	=
+		=

36 + 36		=
(__ + __) +	(_ + _)	=
+		=

42 + 68		=
(__ + __) +	(_ + _)	=
+		=

3. Add mentally, using either strategy - or one of your own!

36 + 38 =

23 + 57 =

27 + 41 =

62 + 35 =

38 + 49 =

22 + 36 =

47 + 34 =

66 + 37 =

27 + 24 =

72 + 19 =

Adding three numbers

The basic strategy is simply break the numbers in parts and add them little by little. You can add all the tens and all the ones separately, or add in whatever parts are easy to add. You can add the two last numbers first or do whatever works!

33 + 28 + 16		=
(30+20+10)	+ 3+8+6	=
60	+ 17	=

27 + 39 + 38		=
(20+30+7+9) +	38	=
50 + 16 +	38	=
66 +	30 + 8	=
	96 + 8	= 104

You can also add first those ones that would form a ten.

15 + 27 + 45		=
(5 + 5 + 10 + 40)	+ 27	=
60	+ 27	=

18 + 52 + 16		=
(8 + 2 + 10 + 50) +	16	=
70 +	16	=

4. Add mentally.

22 + 43 + 38 =

37 + 18 + 15 =

22 + 43 + 38 =

24 + 13 + 25 =

54 + 16 + 22 =

28 + 18 + 36 =

51 + 15 + 16 =

63 + 12 + 17 =

17 + 60 + 20 =

50 + 31 + 19 =

Continue to subtracting 2-digit numbers mentally

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Adding 2-digit numbers mentally Free lesson plan from Homeschool Math.net

Adding 2-digit numbers mentally
Free lesson plan from Homeschool Math.net