M2.3.1 Chapter 1
Equal groups
There are 4 plates with 3 beans on each plate. How many beans is that?
Your guess
Pick one before you read on. It is fine to be wrong.
By the end you can
- Write 4 groups of 3 as 4 × 3
- Show 4 × 3 as rows and columns of dots
Mei puts 3 beans on a plate. She does this for 4 plates.
Every plate has the same number of beans. The plates are equal groups. Equal groups all have the same number of things.
Draw it first
Write 4 groups of 3
1 Draw one group of 3 dots or beans.
2 Draw 3 more groups the same.
3 Write it as 4 × 3 = 12. Say "4 times 3 is 12".
The word times means "groups of". We write it as ×. So 4 × 3 means 4 groups of 3.
To find the answer, add the groups. 3 + 3 + 3 + 3 = 12.
Worked example
Write 3 groups of 5 beans as a times sum. Find the answer.
- Count the groups. There are 3.
- Count the beans in one group. There are 5.
- Write the groups first, then the beans in each: 3 × 5.
- Add the groups: 5 + 5 + 5 = 15.
Answer: 3 × 5 = 15
Now line the groups up under each other. Each group is a row. A row goes across.
Things set out in equal rows are an array. A line going down is a column. This array has 4 rows and 3 columns.
See it three ways
With things
Put 12 stones in 4 rows. Put 3 stones in each row.
As a picture
In symbols
4 × 3 = 12. Also 3 × 4 = 12.
Try it
Make an array of beans
You need: 12 beans, or 12 small stones, a flat table.
- Put 3 beans in a line. That is one row.
- Make 3 more rows the same, one under the other.
- Count all the beans.
- Now walk to the side of the table. Look at the beans again.
What you will see: From the front you see 4 rows of 3. From the side you see 3 rows of 4. It is 12 beans both ways.
A wrong idea many people have
4 × 3 means 4 + 3.
Why it feels right: The numbers are the same. And the × sign looks like a + sign turned on its side.
The test that breaks it: Put 3 beans on each of 4 plates. Count all the beans. Is it 7?
What is true: 4 × 3 means 4 groups of 3, which is 12. Only 4 + 3 is 7.
Key words
- equal groups
- groups that all have the same number of things.
- times
- groups of. 4 × 3 is read "4 times 3".
- row
- a line of things going across.
- array
- things set out in equal rows.
Back to your guess
The answer is B: 12 beans, because it is 4 groups of 3.
There are 4 equal groups of 3. Add them: 3 + 3 + 3 + 3 = 12. We write this as 4 × 3 = 12.
Check yourself
Try each one before you look. A wrong answer tells you where to go next.
Look at the picture. Which sum tells the story?
See the answer
3 × 4 = 12
There are 3 groups with 4 beans in each. 4 + 4 + 4 = 12.
- If you chose B: That adds the 3 groups to the 4 beans. A times sum means 3 groups of 4.
- If you chose C: Each group has 4 beans, not 3. Count the beans in one group.
Kofi has 5 bowls with 3 stones in each bowl. How many stones is that?
See the answer
15 stones
5 bowls of 3 is 5 × 3. Add: 3 + 3 + 3 + 3 + 3 = 15.
- If you chose A: That is 5 + 3. A times sum means 3 stones, five times.
- If you chose B: That is the stones in one bowl. There are 5 bowls, so count 3 five times.
Mei makes 2 rows of 6 dots. Tariq makes 6 rows of 2 dots. Who has more dots?
See the answer
They have the same number.
2 × 6 = 12 and 6 × 2 = 12. Turn one array on its side and you get the other.
- If you chose A: Her rows are longer, but she has fewer of them. Count all the dots.
- If you chose B: He has more rows, but his rows are shorter. Count all the dots.
Look at the picture. Write the array as a times sum.
See the answer
3 × 5 = 15. You can also write 5 × 3 = 15.
There are 3 rows with 5 dots in each row. 5 + 5 + 5 = 15. Turned on its side it is 5 rows of 3.
Draw 5 groups of 2 dots. Write the times sum. How many dots are there?
See the answer
5 × 2 = 10. There are 10 dots.
2 + 2 + 2 + 2 + 2 = 10. That is 5 groups of 2.
Look at Ana's seeds. How many columns does the array have?
See the answer
6 columns
There are 2 rows and 6 seeds in each row. A column goes down. Each row has 6 seeds, so there are 6 columns.
- If you chose A: That is the number of rows. A row goes across. A column goes down.
- If you chose C: That is the number of seeds. A column is a line of seeds going down.
Which number sentence gives the same total as 5 × 2?
See the answer
2 × 5 = 10
5 × 2 = 10. Turn the array on its side. It is 2 rows of 5. 2 × 5 = 10 too.
- If you chose B: You added. 5 × 2 means 5 groups of 2. Add 2 five times.
- If you chose C: You used 5 twice. In 5 × 2 each group has 2, not 5. 2 + 2 + 2 + 2 + 2 = 10.
Keep it
Copy this into your exercise book.
4 × 3 means 4 groups of 3. Add them: 3 + 3 + 3 + 3 = 12. An array sets the groups out in rows. 4 × 3 and 3 × 4 are both 12.
Draw: 12 dots in 4 rows of 3. Write 4 × 3 = 12 under them. Turn the page and write 3 × 4 = 12.