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# Addition and subtraction strategies 1

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Last updated 26 October 2012 15:28 by NZTecAdmin

## The purpose of this activity

In this activity, the learners use strings of beads and/ or number lines to develop addition and subtraction mental partitioning strategies for two-digit by one-digit problems, for example, 27 + 8, 36 – 7.

## The teaching points

• Learners who ‘count on’ (for example, calculating 27 + 8 by counting 27, 28, 29, 30, etc, either using fingers or in their heads) or who can only use a calculator to make calculations will be disadvantaged in completing the numeracy demands of everyday life.
• It is not anticipated that you will teach a learner many different strategies for solving a single problem, but rather you will work with one strategy at a time, noting that different problems lend themselves to different strategies.
• Learners need to know addition and subtraction facts to 10 + 10 and the place value of digits in whole numbers to 100 before undertaking this activity.
• Discuss the strategies and how they can be used with the learners.
• Discuss with the learners the contexts or situations where they need to solve addition and subtraction problems mentally.

Note: This learning sequence can be used with single-digit addition and subtraction sito help the learners with their basic 10 + 10 addition and subtraction facts.

## Resources

• 100 beads, 50 of one colour and 50 of another colour, threaded on to a string in blocks of 10 for each colour or number lines marked in ones.

• Number lines marked in tens.

## The guided teaching and learning sequence

1. Ask the learner(s) to use the string of beads or the number lines marked in ones to solve 27 + 8 in any way they can and explain what they have done. If the learner ‘counts on’, ask if they can see any other way to solve the problem.

• If the learner(s) shows evidence of some partitioning strategy, ask them to demonstrate on the beads and continue to develop that strategy throughout the activity.
• If the learner(s) shows no evidence of using partitioning strategies, encourage them to use the strategy of ‘making tens’ by:
• asking the learner(s) how many beads are needed to get from 27 to 30 and marking + 3 on the number line
• asking how many remain of the 8 and marking + 5 on the number line.

2. Repeat the process with a variety of different two-digit by one-digit addition problems.

3. Once the learner(s) is familiar with the process, ask them to do the activity on the number line marked in tens (0, 10, 20, 30, 40 … 100), for example, starting at 27 on the line and marking + 3 to 30 and + 5 to 35.

4. Repeat the process with a variety of two-digit by one-digit addition problems. 5. Once the learners are familiar with the process, repeat each of the steps for two-digit by one-digit subtraction problems, for example 45 – 7.

5. Once the learners are familiar with the process, repeat each of the steps for two-digit by onedigit subtraction problems, for example 45 – 7.

## Follow-up activity

In pairs, the learners make up and solve two-digit by one-digit addition and subtraction problems, sharing the strategies they use with their partner.

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