Addition/Subtraction strategies

Achievement objectives

NA3-1: Use a range of additive and simple multiplicative strategies with whole numbers, fractions, decimals, and percentages.

Description of mathematics

• Addition and subtraction, AA and AM (stage 6 and 7)

Required materials

See Materials that come with this resource to download:

• Addition subtraction strategies activity (.pdf)
• Addition subtraction strategies activity (.doc)

Activity

Prior knowledge

Experience with the addition and subtraction strategies: jumping on the number line, reversing subtraction problems, tidy numbers, equal additions, using doubles, using multiplicaton, and addition with decimals.

Background

This activity provides students with the opportunity to revise a number of addition and subtraction problems.

Comments on the exercises

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.

Exercise 1

This exercise asks students to solve unknowns in an addition problem by using a 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

This exercise asks students to solve subtraction problems by reversing them.

Exercise 3

This exercise asks students to use tidy numbers. When working with decimals, whole numbers act as tidy numbers.

Exercise 4

This exercise asks students to solve problems when one number is near 100.

Exercise 5

This exercise asks students to solve problems using 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

This exercise asks students to solve problems using doubles.

Exercise 7

This exercise asks students to solve problems using multiplication. It requires knowledge of factors and is a level 4 activity.

Exercise 8

This exercise asks students to use place value strategies to add decimals. This is a level 4 activity. 

The quality of the images on this page may vary depending on the device you are using.