Topics
4. Multiplication and division
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Australia
Year 4

4.01 Finding unknowns

Lesson
Practice
Lesson

Are you ready?

Remembering your  times tables  will help you solve problems in this lesson. Let's give this problem a try.

Examples

Example 1

What is 8\times3?

Worked Solution
Create a strategy

We can think of it as 8 groups of 3.

An array of squares with 8 rows and 3 columns.
Apply the idea

Count the number of squares in the array to get:8\times3=24

Idea summary

We get the same answer whichever way we look at our array.

An image showing an array with 2 columns and 4 rows. Ask your teacher for more infromation.

Find unknowns with multiplication and division

This video shows us how to find unknowns using our multiplication and division facts.

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Examples

Example 2

Complete the statement with the missing number: 8\times⬚=80

Worked Solution
Create a strategy

Rewrite the statement as a division statement.

Apply the idea

We can rewrite the statement as 80\div8=⬚.

To find the answer, we can count up by 8's until we get to 80. We can see that we needed to count up 10 times: 8, \, 16, \, 24, \, 32, \, 40, \, 48, \, 56, \, 64, \, 72, \,80. So: 80\div 8=10 Which also means:8\times10=80

Idea summary

We can write multiplication statements as division statements to help find an unknown.

Turn around facts tell us that for multiplication the order doesn't matter. So for something like 4 \times 5, we can also write it as 5 \times 4 and get the same answer.

Outcomes

ACMNA073

Apply place value to partition, rearrange and regroup numbers to at least tens of thousands to assist calculations and solve problems

ACMNA074

Investigate number sequences involving multiples of 3, 4, 6, 7, 8, and 9

ACMNA075

Recall multiplication facts up to 10 × 10 and related division facts

ACMNA076

Develop efficient mental and written strategies and use appropriate digital technologies for multiplication and for division where there is no remainder

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