# Addition/Subtraction Strategies

Teacher’s Notes

These exercises and activities are for students to use independently of the teacher to practice number properties and strategies for addition and subtraction.

### Number Framework domain and stage:

Addition and subtraction 6 to 7. Advanced additive to advanced multiplicative.

### Curriculum reference:

Number, level 3

### Numeracy Project book reference:

These activities can be used to follow the teaching of addition (book 5) and are designed to be used as revision/maintenance for students who are able to use the associated number properties with decimals.

### Prior knowledge. Students should be able to:

- recall all their addition and subtraction facts to 20 (at least)
- be able to use the relevant number strategy with whole numbers
- explain place value in relation to decimals, and identify ‘which decimal is larger’

### Background

Each exercise focuses on one add/sub strategy. The exercise starts with small numbers and extends to big numbers and decimals. The last question in each exercise asks students to write a word or story problem. In the final two exercises students are choosing the most appropriate strategy. The amount of working students show will be individual. This is a good opportunity for the teacher to discuss with students how much working is appropriate to show how the questions have been answered.

Note that the format for four digit numbers involves the use of a comma to separate the thousands. For example, 2, 306

Exercise 1: Jumping the number line Students should already know that 6 + ? = 8 and ? + 6 = 8 can both be solved the same way, as addition is commutative. It may pay to discuss whether or not this can be extended to subtractions, for a number of students incorrectly assume it can. This also applies to exercise 3, if that exercise is used before exercise 1

Exercise 2: Don’t subtract - add

Exercise 3: Using tidy numbers When working with decimals, whole numbers act as tidy numbers. Students should already know that 6 + ? = 8 and ? + 6 = 8 can both be solved the same way, as addition is commutative. It may pay to discuss whether or not this can be extended to subtractions, for a number of students incorrectly assume it can.

Exercise 4: When one number is near 100

Exercise 5: Equal Additions When working with decimals, whole numbers act as tidy numbers. Also note that in questions 11 and 12 the problems have been turned into equations. This change in pattern can confuse some students, who do not realise the problems are the same. It may be worthwhile discussing what the equals sign means in relation to these questions. Earlier problems do not have an equals sign as students often erroneously learn that it means ‘work out the answer’

Exercise 6: Near doubles

Exercise 7: Using multiplication

Exercise 8: Mixing the methods

### Practice exercises with answers:

### Related activities:

Loopy: Multiple ways to add and subtract

Students need to know that the answer is given in the form a x b, and will need to be able to look for a common factor. For example, the answer to 21 + 15 is given as 12 x 3 (because 21 = 7 x 3 and 15 is 5 x 3)

PDF (77KB) or Word (49KB)

A selection of add/sub loopies for students to test their strategies/knowledge.

PDF (41KB) or Word (65KB)

Refer to the ‘other materials’ section for information on how to use a set of loopy cards.