Math Core

Eggs in a carton, seats in a theater and windows on a building are all lined up in neat rows. When things are arranged like this, multiplication makes counting them fast.

What is an array?

Definition

Array

An array is a set of objects arranged in rows and columns. Every row has the same number of objects.

Rows go across, from left to right. Columns go up and down.

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This array has 3 rows. Each row has 5 squares. So it also has 5 columns.

Arrays are equal groups

Each row is one equal group. So an array is just equal groups lined up neatly.

3 rows of 5 means 3 groups of 5:

3 \times 5 = 15

You can check by skip counting the rows: 5, 10, 15.

Multiplying with an array

\text{rows} \times \text{number in each row} = \text{total}

In this unit, we write the number of rows first.

Drawing an array

To show $2 \times 6$ with an array, draw 2 rows with 6 in each row.

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Skip count by sixes: 6, 12. So $2 \times 6 = 12$ .

Worked example: Count an array

Write a multiplication for this array. Then find the total.

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There are 4 rows. Each row has 3 flowers.

$4 \times 3$ . Skip count by threes: 3, 6, 9, 12.

$4 \times 3 = 12$ flowers.

Worked example: Seats in rows

A classroom has 5 rows of desks. There are 4 desks in each row. How many desks are there?

The desks make an array with 5 rows of 4.

$5 \times 4$ . Skip count by fours: 4, 8, 12, 16, 20.

There are 20 desks.

Worked example: Find the missing number

An array has 18 stickers. There are 3 rows. How many stickers are in each row?

You need $3 \times ? = 18$ . Try skip counting by different numbers, 3 times each.

By fives: 5, 10, 15. Too small.
By sixes: 6, 12, 18. Yes!

Each row has 6 stickers.

Common mistake

Don't count the rows twice or mix rows and columns. Rows go across. For the sunflower array above, there are 4 rows going across, and each row has 3 flowers. Count one row carefully before you multiply.

Tip

Arrays make it easy to check your work. Count one row, count the number of rows, then skip count. You never have to count every object one by one.

Practice

Practice 1

How many squares are in this array?

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Type a number.

Practice 2

How many rows does this array have?

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Type a number.

Practice 3

Which multiplication matches this array?

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Practice 4

An array has 3 rows with 6 cans in each row. How many cans are there?

Type a number.

Practice 5

A garden has 5 rows of carrots. Each row has 7 carrots. How many carrots are in the garden?

Type a number.

Practice 6

Which array shows $2 \times 3$ ?

Practice 7

A muffin pan has 24 cups. The cups are in rows of 6. How many rows are there?

Type a number.

Practice 8

Ana makes an array of 2 rows with 8 beads in each row. Then she adds 1 more row of 8. How many more beads does she have now than before?

Type a number.