Multiplying with Decimals

Multiplying with Decimals

Our friend: The place value chart

We've already learned how to multiply larger numbers, and how to add and subtract decimals, but now let's learn to multiply with decimals.

Here we have a decimal.

Remember our place values!

Multiple Choice

In 2.34, which statement is correct?

• The 2 is in the tens place

• The three is in the ones place

• The 2 is in the ones place

• The 4 is in the tenths place

Now we have 1.2.

We're going to ignore the decimal in these numbers, so 1.2 becomes 12 and 2.34 becomes 234.

To multiply numbers with decimals, we multiply as usual.

Just ignore the decimal signs for now!

Using the area model

Or if you are confident in the traditional method of multiplying, that's okay too!

Multiple Choice

What's the result of 234x12?

• 5123

• 6363

• 3123

• 2808

Great! You found the answer!

But what about our decimals???

Let's take a step back and look at each individual multiplier.

We're not going to forget about our answer, but we'll come back to it.

Look at how many digits are after the decimal sign.

The numbers after the decimal sign is "3" and "4."

Multiple Choice

How many decimal places does 2.34 have?

• 1

• 2

• 3

• 4

There are two numbers.

Keep that in mind!

Multiple Choice

What about in 1.2?

• 1

• 2

• 3

What about in 1.2?

There is only one.

How many decimal places did we have in total?

Let go back to our answer to 2.34 x 1.2, which was 2808!

We need 3 decimal places in our answer of 2808.

So our 2808 becomes 2.808.

Let's do another example.

We're going to count the decimal places.

Multiple Choice

In 8.2, how many decimal places are there?

• 1

• 2

• 3

Multiple Choice

In 5.5, how many decimal places are there?

• 1

• 2

• 3

Multiple Choice

8.2 x 5.5=4510 without decimals. What is the correct answer WITH decimals?

• 451.0

• 45.10

• 4.510

Multiple Choice

How many decimal places are in 4.331?

• 1

• 2

• 3

Multiple Choice

How many decimal places are in 9.21?

• 1

• 2

• 3

Multiple Choice

If 4.331 x 9.21= 3988851 without decimals, WITH decimals, the answer should be...

• 39.88851

• 3988.851

• 3.988851