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Level Three > Number and Algebra

# When Subtraction becomes Addition

Achievement Objectives:

Specific Learning Outcomes:

Solve subtraction problems by using addition.

Description of mathematics:

Number Framework Stage 6

Required Resource Materials:
Strips of paper
Calculators
Activity:

### Using Materials

Problem: “Ana goes Christmas shopping. She spends \$345.60 on toys for her children and comes home with \$208.95. How much money did she have to start with?”

Write ? – 345.60 = 208.95 on the board. Have the students cut a strip of paper, representing Ana’s unknown starting amount.

Cut off a piece and write \$345.60 on it.

Discuss what goes on the other piece. (Answer: \$208.95.)

Discuss what the starting amount must have been. (Answer: \$345.60 + \$208.95 = \$554.55.)

Examples: Word stories and recording for: ? – 345 = 789

? – 456 = 102

? – 67.9 = 43.19

? – 67 = 90

? – 1 000 = 3 000

? – 200 = 560 ...

### Using Imaging

Examples: Imagine the strips to solve these problems: Word stories and recording for:

? – 200 = 600

? – 220 = 230

? – \$7.90 = \$3.10

? – 111 = 238

? – 10 000 = 4 500 ...

### Using Number Properties

Examples: Don’t use calculators for the fraction problems. Word stories and recording for:

? – 212.98 = 600.0034

? – 200.08 = 45.89

? – 7.9909 = 3.1091

? – 10 = 3/4

? – 2 1/3 = 1/3

? – 0.04 = 0.0009 ...

### Understanding Number Properties:

a and b stand for any numbers: â–¡ a = b. How would you work out

the number to put in the box? (Answer: I would work out the answer to a + b.)

## Reversing Addition

Solve subtraction problems by using addition.

## You Don't Need the Number

Solve addition problems by using compatible numbers.

## Close to 100

Recall the number of tens and hundreds in 100s and 1000s.

Identify symbols for any fraction, including tenths, hundredths, thousandths, and those greater than 1.

Solve addition and subtraction problems using decomposition, leading to a written algorithm

## A Standard Written Form for Addition

Solve addition and subtraction problems using decomposition, leading to a written algorithm.

## Multiple Ways to Add and Subtract

Use multiplication to solve addition and subtraction problems.