Adding and Subtracting Whole Numbers

Benchmark MA.2.A.2.1: Recall basic addition and

related subtraction facts. MA.2.A.2.2: Add and subtract multi-digit

whole numbers through three digits with fluency by using a variety of strategies, including invented and standard algorithms and explanations of those procedures.

Objectives Review of basic subtraction facts. Develop fluency with multi-digit addition and

subtraction.

Subtraction

Subtraction Facts Subtraction is removing some objects from a

group. The meaning of 5 – 3 = 2 is "Three objects are

taken away from a group of five objects and two objects remain".

Examples: 6 - 2 = 4 9 - 3 = 6

Subtraction Example Example: 8 - 3

This is the subtraction of 3 from 8 or 8 minus 3

Subtraction is represented as shown: 8 - 3

We can count down to get the difference8 7, 6, 5 so the difference is 5

3

What is Subtraction?

Subtracting whole numbers is the inverse operation of adding whole numbers.

Relationship between Addition and Subtraction There is an inverse relationship between

addition and subtraction. If a math fact is considered, for example 3 + 7

= 10. Then the following are also true:

10 - 3 = 7 10 - 7 = 3

Similar relationships exist for subtraction, for example 10 - 3 = 7. Then the following are also true: 3 + 7 = 10 7 + 3 = 10

Subtraction word problem Mika is 6 years younger than her cousin Ben.

Ben is 11 years old, how old is Mika?

A.Mika is 5 years old B.Mika is 6 years old C.Mika is 4 years old D.Mika is 7 years old

Subtraction factsSubtractions with one digit are usually fairly easy.

Things start getting complicated when you have more than one digit and you cannot remove the number at the bottom from the number on top such as when doing 85 − 8

Two-Digit Subtraction What is 76 – 23? Let’s use the 100 Chart

Two-Digit Subtraction (cont.) What is 76 – 23? Start at 67. 23 is 2 tens 3 ones Let’s count back in the 100 chart 2 tens and

then 3 ones

Two-Digit Subtraction (cont.)

Examples 83 – 50 = ____

22 – 14 = ____

46 – 15 = ____

31 – 15 = ____

73 – 26 = ____

Word Problem There were 32 birds in the trees. Then 15

birds flew away. How many birds are in the trees now?

Break Apart Approach Breaking Apart makes subtraction easier. Example:

63 – 7 = ___ 7

Subtract 3 to get 60. Then subtract 4 more. 63 – 7 = 63 – 3 – 4 = 60 – 4 = 56

3 4

Examples Use the Break Apart strategy to find the

difference: 75 – 7 = _____

82 – 5 = _____

76 – 9 = _____

85 – 8 = _____

Subtraction by Regrouping What is 42 – 15?

Step1: Are they enough ones to subtract 5?

No. Step 2: Regroup 1 ten as 10 ones.

Subtraction by Regrouping (cont.) Step3: Subtract the ones.

12 – 5 = 7

Step 4: Subtract the tens.3 – 1 = 2

Subtraction by Regrouping (cont.) The difference is then:

27

Example

Since you could not remove 8 from 5, you borrowed a ten from 8 tens and add that to 5 to make it 15

Word Problem Jane and her mom made 33 puppets for the

craft fair. By noon, they had sold 14 of the puppets. How many puppets did they have left to sell?

Solve the Following Problems

89- 25

69- 35

50- 9

99- 12