Finding the difference 32

unit

Finding the difference 32

NSW Syllabus Solve simple addition and subtraction problems using a range of efficient mental and written strategies

Lesson focus Explore subtraction as `finding the difference'

Materials ? dice ? BLM 3 (Number cards 0?10) ? calculators ? blank cards

Getting started

Brainstorm what subtraction means. Identify the many different ways that subtraction is implied. (For example, take away situations, comparison situations, missing addend situations, find the difference situations.)

Record the students' answers on the board or on a large piece of paper. Add a simple example problem for each.

Refer to the Teacher Dashboard for resources such as videos that address potential difficulties, digital teaching objects (interactives), blackline masters and investigations.

SB M&H

Page 133

SB M&H APM

Page 65

Learning activities

1 In pairs, one student tosses two dice together and finds the difference between the two numbers thrown. The student records this number and the other student tosses the dice and does the same thing. After five turns each, students add up the five numbers they have written down. The student with the highest score wins.

2 Each pair of students places a set of number cards labelled 0 to 9 (BLM 3) face down between them. Both students start with a score of 0. They take two cards each and arrange them to make the largest two-digit number they can. They compare numbers and together subtract the smaller from the larger number to find the difference. The student who made the larger number adds the difference to their score. They continue until one student reaches a score of 100. Students can use a calculator to help keep score.

3 Give each student a blank card and ask them to record a number between 1 and 30 on it. Students sit in a circle and place their card face down in front of them. Call the names of two students at a time. These students must hold up their card and then try to be the first to work out the difference between the two numbers and say the answer. Repeat for other pairs of students.

Support activities

Reinforce that finding the difference is the same as subtracting so any of the strategies previously discussed will help students work out the answer.

Tell students that the difference between two numbers is 3. Ask them to record possible pairs of numbers.

Extension activities

Ask students to write subtractions where the difference between the numbers is 8 (or another number).

Tell students that the difference between two numbers is 15. Ask: What might the two numbers be?

Reflection

Review the various subtraction strategies that students have used to find the difference between numbers. Invite different students to explain, and demonstrate if necessary, their method.

Assessment

Can students interpret `find the difference' as a subtraction situation?

Can students choose the most appropriate solution strategy for the question involved?

The Assessment Book is best used for end-of-year assessment; however, appropriate questions on page 5 can be used to check students' understanding of difference.

9780190319267 ? Oxford University Press 2020

133

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